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

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

//examples/cpp:hello-success_test                                        PASSED in 0.1s
//examples/java-native/src/test/java/com/example/myproject:hello         PASSED in 0.3s

Executed 2 out of 2 tests: 2 tests pass.
There were tests whose specified size is too big. Use the --test_verbose_timeout_warnings command line option to see which ones these are.
INFO: Build completed successfully, 20 total actions
installing
post-installation fixup
patching script interpreter paths in /nix/store/9akfj738h56c6a87rk69s6k31ly0zglc-bazel-0.15.2
/nix/store/9akfj738h56c6a87rk69s6k31ly0zglc-bazel-0.15.2

//examples/java-native/src/test/java/com/example/myproject:hello         PASSED in 0.7s

Executed 2 out of 2 tests: 2 tests pass.
There were tests whose specified size is too big. Use the --test_verbose_timeout_warnings command line option to see which ones these are.
INFO: Build completed successfully, 22 total actions
installing
post-installation fixup
patching script interpreter paths in /nix/store/14xqf8q9sgkxh9z1018c1lsfqyyhawax-bazel-0.15.2
checking for references to /tmp/.bazel-1000 in /nix/store/14xqf8q9sgkxh9z1018c1lsfqyyhawax-bazel-0.15.2...
/nix/store/14xqf8q9sgkxh9z1018c1lsfqyyhawax-bazel-0.15.2