a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnsupportedSystem = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnsupportedSystem = true; }
to ~/.config/nixpkgs/config.nix.

+ ocamlfind ocamlopt hack/fsevents/fsevents_stubs.o src/common/xx/xx_stubs.o hack/heap/hh_shared.o hack/utils/get_build_id.o hack/utils/sys/files.o hack/utils/sys/getrusage.o hack/utils/sys/handle_stubs.o hack/utils/sys/nproc.o hack/utils/sys/priorities.o hack/utils/sys/processor_info.o hack/utils/sys/realpath.o hack/utils/sys/sysinfo.o src/third-party/lz4/lz4.o src/third-party/lz4/lz4frame.o src/third-party/lz4/lz4hc.o src/third-party/lz4/xxhash.o hack/utils/get_build_id.gen.o -cclib -l -cclib pthread -cclib -framework -cclib CoreServices -cclib -framework -cclib CoreFoundation -linkpkg -package unix -package str -package bigarray -package sedlex -package wtf8 -package ppx_deriving -package dtoa -package lwt_ppx -package lwt.unix -package lwt_log -package lwt -I hack/fsevents -I hack/third-party/core -I hack/utils -I hack/fsnotify_darwin -I hack/third-party/avl -I hack/utils/collections -I hack/dfind -I hack/injection/default_injector -I hack/stubs -I hack/utils/disk -I hack/utils/sys -I hack/find -I hack/heap -I hack/utils/hh_json -I hack/procs -I src/common/lwt -I hack/socket -I hack/utils/build_mode/prod -I hack/watchman -I src/parser -I src/common/utils -I src/common -I src/common/lints -I src/monitor -I src/stubs -I src/server/protocol -I src/server/server_files -I src/commands -I src/commands/config -I src/common/xx -I src/common/profiling -I src/common/errors -I src/common/modulename -I src/common/ty -I src/flowlib -I src/server/server_utils -I src/server/shmem -I src/common/span -I src/parser_utils -I src/typing -I src/common/monad -I src/parser_utils/output -I src/third-party/ocaml-vlq/src -I src/third-party/ocaml-sourcemaps/src -I src/services/autocomplete -I src/common/audit -I src/server/persistent_connection -I src/common/tarjan -I src/common/transaction -I src/parser_utils/output/printers -I src/state/heaps/parsing -I src/parsing -I src/server/env -I src/state/heaps/module -I src/state/locals/module -I src/services/inference/module -I src/services/inference -I src/state/heaps/context -I src/server/command_handler -I src/server/error_collator -I src/server/find_refs -I src/server/monitor_listener -I src/server/rechecker -I src/server/lazy_mode_utils -I src/services/flowFileGen -I src/services/port -I src/services/saved_state -I src/services/type_info -I src/server -I src/monitor/logger -I src/monitor/connections -I src/monitor/utils -I src hack/fsevents/fsevents.cmx hack/third-party/core/commutative_group.cmx hack/third-party/core/container.cmx hack/third-party/core/core_printf.cmx hack/third-party/core/hack_caml.cmx hack/third-party/core/monad.cmx hack/third-party/core/polymorphic_compare.cmx hack/third-party/core/poly.cmx hack/third-party/core/core_list.cmx hack/utils/hh_core.cmx hack/fsnotify_darwin/fsnotify.cmx hack/third-party/avl/monoidAvl.cmx hack/utils/collections/myMap_sig.cmx hack/utils/collections/myMap.cmx hack/utils/collections/stringKey.cmx hack/utils/collections/sMap.cmx hack/utils/collections/sSet.cmx hack/dfind/dfindEnv.cmx hack/dfind/dfindMaybe.cmx hack/dfind/dfindAddFile.cmx hack/injection/default_injector/injector_config.cmx hack/stubs/eventLogger.cmx hack/third-party/core/option.cmx hack/utils/collections/intKey.cmx hack/utils/collections/iMap.cmx hack/utils/collections/iSet.cmx hack/utils/collections/priorityQueue.cmx hack/utils/disk/disk_sig.cmx hack/utils/disk/realDisk.cmx hack/utils/disk/testDisk.cmx hack/utils/disk/disk.cmx hack/utils/string_utils.cmx hack/utils/hh_logger.cmx hack/utils/utils.cmx hack/utils/sys/sys_utils.cmx hack/utils/sys/timer.cmx hack/utils/sys/timeout.cmx hack/utils/marshal_tools.cmx hack/utils/sys/pidLog.cmx hack/utils/sys/fork.cmx hack/utils/sys/handle.cmx hack/utils/sys/daemon.cmx hack/utils/sys/path.cmx hack/dfind/dfindServer.cmx hack/dfind/dfindLib.cmx hack/utils/marshal_tools_lwt.cmx hack/dfind/dfindLibLwt.cmx hack/find/find.cmx hack/heap/ident.cmx hack/heap/prefix.cmx hack/heap/value.cmx hack/heap/workerCancel.cmx hack/utils/hh_json/hh_json.cmx hack/utils/measure.cmx hack/utils/stats.cmx hack/heap/sharedMem.cmx hack/procs/bucket.cmx hack/utils/exit_status.cmx hack/utils/sys/printSignal.cmx hack/procs/worker.cmx hack/procs/workerController.cmx hack/procs/multiThreadedCall.cmx hack/procs/multiWorker.cmx hack/procs/workerControllerLwt.cmx src/common/lwt/lwtUtils.cmx hack/procs/multiWorkerLwt.cmx hack/utils/opaqueDigest.cmx hack/socket/socket.cmx hack/third-party/core/core_result.cmx hack/third-party/core/fn.cmx hack/utils/buffered_line_reader_sig.cmx hack/utils/buffered_line_reader.cmx hack/utils/buffered_line_reader_lwt.cmx hack/utils/build_id.cmx hack/utils/build_mode/prod/build_mode.cmx hack/utils/collections/immQueue.cmx hack/utils/file_content.cmx hack/utils/file_url.cmx hack/utils/hh_json/hh_json_helpers.cmx hack/utils/http_lite.cmx hack/utils/jsonrpc.cmx hack/utils/lsp.cmx hack/utils/lsp_fmt.cmx hack/utils/lsp_helpers.cmx hack/utils/sys/lock.cmx hack/utils/sys/tmp.cmx hack/utils/sys/tty.cmx hack/watchman/watchman_sig.cmx hack/watchman/watchman.cmx hack/watchman/watchman_lwt.cmx src/parser/file_key.cmx src/parser/loc.cmx src/common/utils/utils_js.cmx src/common/path_matcher.cmx src/common/files.cmx src/common/flow_version.cmx src/common/lints/lints.cmx src/common/lints/severity.cmx src/common/lints/lintSettings.cmx src/common/lints/strictModeSettings.cmx src/common/verbose.cmx src/parser/ast.cmx src/common/options.cmx src/monitor/fileWatcherStatus.cmx src/stubs/flowEventLogger.cmx src/monitor/serverStatus.cmx src/server/protocol/socketHandshake.cmx src/server/server_files/server_files_js.cmx src/commands/commandConnectSimple.cmx src/commands/commandMeanKill.cmx src/common/flowExitStatus.cmx src/commands/commandConnect.cmx src/commands/commandSpec.cmx src/commands/config/version_regex.cmx src/common/semver.cmx src/common/xx/xx.cmx src/commands/config/flowConfig.cmx src/common/profiling/profiling_js.cmx src/common/reason.cmx src/common/utils/listUtils.cmx src/common/utils/nel.cmx src/common/utils/optionUtils.cmx src/common/errors/errors.cmx src/common/modulename/modulename.cmx src/common/ty/ty.cmx src/flowlib/flowlib.cmx src/server/server_utils/file_input.cmx src/server/protocol/serverProt.cmx src/server/protocol/persistent_connection_prot.cmx src/server/protocol/monitorProt.cmx src/monitor/monitorRPC.cmx src/server/shmem/sharedMem_js.cmx src/commands/commandUtils.cmx src/common/span/spanMap.cmx src/common/lints/exactCover.cmx src/common/errors/error_suppressions.cmx src/common/utils/tree.cmx src/common/utils/utils_jsx.cmx src/parser/ast_utils.cmx src/parser/parse_error.cmx src/parser/lex_env.cmx src/parser/token.cmx src/parser/lex_result.cmx src/parser/lexer.cmx src/parser/parser_env.cmx src/parser_utils/flow_ast_mapper.cmx src/parser_utils/scope_api.cmx src/parser_utils/ssa_api.cmx src/typing/key.cmx src/typing/changeset.cmx src/typing/key_map.cmx src/typing/type.cmx src/typing/trace.cmx src/typing/constraint.cmx src/typing/existsCheck.cmx src/typing/graph_explorer.cmx src/typing/scope.cmx src/typing/type_table.cmx src/typing/context.cmx src/typing/default.cmx src/typing/flow_error.cmx src/typing/debug_js.cmx src/typing/obj_type.cmx src/typing/tvar.cmx src/typing/react_kit.cmx src/typing/repos_cache.cmx src/typing/speculation.cmx src/typing/type_filter.cmx src/typing/type_inference_hooks_js.cmx src/typing/type_mapper.cmx src/typing/type_visitor.cmx src/typing/flow_js.cmx src/typing/env.cmx src/typing/abnormal.cmx src/typing/refinement.cmx src/typing/typed_ast.cmx src/typing/destructuring.cmx src/typing/func_params.cmx src/typing/func_sig.cmx src/typing/class_sig.cmx src/typing/import_export.cmx src/typing/type_annotation.cmx src/typing/statement.cmx src/parser/parser_common.cmx src/parser/type_parser.cmx src/parser/declaration_parser.cmx src/parser/translator_intf.cmx src/parser/estree_translator.cmx src/parser/pattern_cover.cmx src/parser/expression_parser.cmx src/parser/jsx_parser.cmx src/parser/object_parser.cmx src/parser/pattern_parser.cmx src/parser/statement_parser.cmx src/parser/parser_flow.cmx src/parser/token_translator.cmx src/parser_utils/json_of_estree.cmx src/commands/astCommand.cmx src/common/docblock.cmx src/common/monad/stateResult.cmx src/common/monoid.cmx src/common/monad/writer.cmx src/common/ty/ty_debug.cmx src/parser_utils/output/layout.cmx src/parser_utils/output/js_layout_generator.cmx src/parser_utils/output/source_map_config.cmx src/third-party/ocaml-vlq/src/vlq.cmx src/third-party/ocaml-sourcemaps/src/sourcemap.cmx src/parser_utils/output/source.cmx src/common/ty/ty_printer.cmx src/common/ty/ty_visitor.cmx src/common/ty/ty_utils.cmx src/common/utils/line.cmx src/services/autocomplete/autocomplete_js.cmx src/typing/ty_normalizer.cmx src/services/autocomplete/autocompleteService_js.cmx src/commands/autocompleteCommand.cmx src/common/audit/expensive.cmx src/common/diff.cmx src/server/persistent_connection/persistent_connection.cmx src/common/flow_lsp_conversions.cmx src/common/loggingUtils.cmx src/common/lwt/lwtInit.cmx src/common/lwt/lwtLoop.cmx src/common/tarjan/tarjan.cmx src/common/transaction/transaction.cmx src/common/ty/ty_serializer.cmx src/common/utils/sMapUtils.cmx src/common/utils/unionFind.cmx src/parser_utils/flow_ast_visitor.cmx src/parser_utils/hoister.cmx src/parser_utils/scope_builder.cmx src/parser_utils/file_sig.cmx src/parser_utils/flow_ast_differ.cmx src/parser_utils/output/printers/pretty_printer.cmx src/parser_utils/output/printers/ast_diff_printer.cmx src/parser_utils/package_json.cmx src/parser_utils/ssa_builder.cmx src/state/heaps/parsing/parsing_heaps.cmx src/parsing/parsing_service_js.cmx src/server/env/checkedSet.cmx src/server/env/serverEnv.cmx src/state/heaps/module/module_heaps.cmx src/state/locals/module/module_hashtables.cmx src/services/inference/module/module_js.cmx src/services/inference/dep_service.cmx src/services/inference/inference_utils.cmx src/typing/gc_js.cmx src/typing/sigHash.cmx src/typing/type_inference_js.cmx src/typing/merge_js.cmx src/state/heaps/context/context_heaps.cmx src/services/inference/init_js.cmx src/typing/sort_js.cmx src/services/inference/merge_stream.cmx src/stubs/flow_server_profile.cmx src/services/inference/merge_service.cmx src/services/inference/types_js.cmx src/server/command_handler/getDef_js.cmx src/server/error_collator/errorCollator.cmx src/server/find_refs/findRefsTypes.cmx src/server/monitor_listener/serverMonitorListenerState.cmx src/server/rechecker/rechecker.cmx src/server/lazy_mode_utils/lazy_mode_utils.cmx src/server/find_refs/findRefsUtils.cmx src/server/find_refs/importExportSymbols.cmx src/server/find_refs/propertyAccessSearcher.cmx src/server/find_refs/variableFindRefs.cmx src/server/find_refs/propertyFindRefs.cmx src/server/find_refs/findRefs_js.cmx src/server/command_handler/refactor_js.cmx src/typing/codegen.cmx src/services/flowFileGen/flowFileGen.cmx src/services/port/comments_js.cmx src/services/port/port_service_js.cmx src/services/saved_state/saved_state.cmx src/services/type_info/suggest.cmx src/typing/query_types.cmx src/services/type_info/type_info_service.cmx src/server/command_handler/commandHandler.cmx src/server/monitor_listener/serverMonitorListener.cmx src/server/serverWorker.cmx src/server/serverEnvBuild.cmx src/server/server_daemon.cmx src/stubs/eventLoggerLwt.cmx src/server/server.cmx src/commands/checkCommands.cmx src/commands/checkContentsCommand.cmx src/commands/commandInfo.cmx src/commands/coverageCommand.cmx src/commands/cycleCommand.cmx src/commands/dumpTypesCommand.cmx src/commands/findModuleCommand.cmx src/commands/findRefsCommand.cmx src/commands/forceRecheckCommand.cmx src/commands/lsCommand.cmx src/commands/genFlowFilesCommand.cmx src/commands/getDefCommand.cmx src/commands/getImportsCommand.cmx src/common/utils/json_rpc.cmx src/commands/ideCommand.cmx src/commands/initCommand.cmx src/commands/lspCommand.cmx src/commands/portCommand.cmx src/commands/refactorCommand.cmx src/commands/saveStateCommand.cmx src/common/lwt/lwtSysUtils.cmx src/monitor/logger/flowServerMonitorLogger.cmx src/monitor/connections/flowServerMonitorConnection.cmx src/monitor/connections/ephemeralConnection.cmx src/monitor/connections/persistentConnection.cmx src/monitor/connections/serverConnection.cmx src/monitor/flowServerMonitorOptions.cmx src/monitor/fileWatcher.cmx src/monitor/flowServerMonitorDaemon.cmx src/monitor/persistentConnectionMap.cmx src/monitor/requestMap.cmx src/monitor/statusStream.cmx src/monitor/utils/exitSignal.cmx src/monitor/flowServerMonitorServer.cmx src/monitor/socketAcceptor.cmx src/monitor/flowServerMonitor.cmx src/commands/serverCommand.cmx src/commands/shellCompleteCommand.cmx src/commands/startCommand.cmx src/commands/statusCommands.cmx src/commands/stopCommand.cmx src/parser_utils/output/flow_prettier_comments.cmx src/commands/suggestCommand.cmx src/commands/typeAtPosCommand.cmx src/commands/versionCommand.cmx src/stubs/extra_commands.cmx src/flow.cmx -o src/flow.native
ocamlfind: [WARNING] Package `threads': Linking problems may arise because of the missing -thread or -vmthread switch
mkdir -p bin
cp _build/src/flow.native bin/flow
installing
post-installation fixup
strip is /nix/store/251fvx4mx9q4zarbca5cciasmn66p668-cctools-binutils-darwin/bin/strip
stripping (with command strip and flags -S) in /nix/store/nlqs6x41bhlg8cbkfl1vih4gm2jn1wz0-flow-0.78.0/bin
patching script interpreter paths in /nix/store/nlqs6x41bhlg8cbkfl1vih4gm2jn1wz0-flow-0.78.0
/nix/store/nlqs6x41bhlg8cbkfl1vih4gm2jn1wz0-flow-0.78.0

cp _build/src/flow.native bin/flow
installing
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/przl0fjhg7zmnkzs6ax6n2h4cmpmcc85-flow-0.78.0
shrinking /nix/store/przl0fjhg7zmnkzs6ax6n2h4cmpmcc85-flow-0.78.0/bin/flow
strip is /nix/store/gpc2wld1s0c6qzx9326cwn1wcx29xzsj-binutils-2.30/bin/strip
stripping (with command strip and flags -S) in /nix/store/przl0fjhg7zmnkzs6ax6n2h4cmpmcc85-flow-0.78.0/bin
patching script interpreter paths in /nix/store/przl0fjhg7zmnkzs6ax6n2h4cmpmcc85-flow-0.78.0
checking for references to /build in /nix/store/przl0fjhg7zmnkzs6ax6n2h4cmpmcc85-flow-0.78.0...
/nix/store/przl0fjhg7zmnkzs6ax6n2h4cmpmcc85-flow-0.78.0