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.

make[1]: Leaving directory '/build/source/hack_parallel'
dune build @install -j auto --profile release
File "dune", line 2, characters 13-41:
2 | (copy_files# external/taint/test/*.ml{,i})
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: cannot find directory: external/taint/test
make: *** [Makefile:32: release] Error 1
builder for '/nix/store/82yag87ycyhbr97gb4mzny10d4mqxjnl-pyre-0.0.11.drv' failed with exit code 2
cannot build derivation '/nix/store/xh48y0cpp06dqw6b3vf6hsvqw73xr2dm-pyre-check-0.0.11.drv': 1 dependencies couldn't be built
error: build of '/nix/store/xh48y0cpp06dqw6b3vf6hsvqw73xr2dm-pyre-check-0.0.11.drv' failed

-> stderr:
Fatal error: exception (runtime-lib/runtime.ml.E "comparison failed"
  ((switch true) vs (switch false) (Loc src/config/discover.ml:205:15)))
Raised at file "src/configurator.ml", line 511, characters 13-22
Called from file "src/config/discover.ml", line 136, characters 2-1023
make: *** [Makefile:5: default] Error 1
builder for '/nix/store/6dh8xxpdcx9n0h5jhn1nhvhwbyv0pgyg-ocaml4.06.1-core-0.11.2.drv' failed with exit code 2
cannot build derivation '/nix/store/qi7irnp5dq96av6j8q9j2qab5wdxlbdd-pyre-0.0.11.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/yf52q748f17ch10yn9794lrgdjhgpl6n-pyre-check-0.0.11.drv': 1 dependencies couldn't be built
error: build of '/nix/store/yf52q748f17ch10yn9794lrgdjhgpl6n-pyre-check-0.0.11.drv' failed