lib: Include darwin in isUnix

[corngood]

Motivation for this change

This should fix darwin package evaluation, which was broken by 4ac1901

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[mention-bot]

@corngood, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Ericson2314 to be a potential reviewer.

[corngood]

See #26021

[vcunat]

Seems OK at a superficial look & quick testing. Let me not keep Hydra waiting.

[corngood]

I don't have a darwin machine to test on, but

before:

➜  nixpkgs git:(master) nix-instantiate -A readline --argstr system x86_64-darwin
error: Package ‘readline-6.3p08’ in ‘/cygdrive/c/Users/David/git/nixpkgs/pkgs/development/libraries/readline/6.3.nix:38’ is not supported on ‘x86_64-darwin’, refusing to evaluate.

after:

➜  nixpkgs git:(darwin-fix) nix-instantiate -A readline --argstr system x86_64-darwin
warning: you did not specify ‘--add-root’; the result might be removed by the garbage collector
/nix/store/v2irshgcwdh3gf8hj60ahkz1z80h8vns-readline-6.3p08.drv

[Ericson2314]

https://github.com/NixOS/nixpkgs/blob/master/lib/tests/systems.nix this should have blown up a long time ago and caught this. @nbp perhaps I used your module test stuff incorrectly?

[Ericson2314]

See also https://github.com/NixOS/nixpkgs/blob/master/lib/tests/release.nix#L28

[nbp]

@Ericson2314 Are you sure the runTests function used in misc.nix and systems.nix tests are actually running anything? I think you might be using, and that the nix-instantiate command does not do what you expect.

nixpkgs/nixos/lib/testing.nix

Line 40 in 5d5c0d5

runTests = driver:

It might be worth to add an expected failing test, to check that the runTests does catch simple failures.