we need a pkgsCross smoke test

[ghost]

Until recently, nixpkgs' cross-compilation infrastructure was considered a sort of exotic/best-effort thing, without much CI or testing until things got to Hydra. That's no longer the case, and we are starting to depend on pkgsCross for increasingly important stuff.

We need a package-sized (i.e. not needing hydra-eval-jobs) build that people can run when refactoring our cross-compilation infrastructure.

When I wrote #238154 I tested it thouroughly on all the targetPlatforms I encounter on a regular basis, but since I don't use LLVM much or Windows at all I neglected to try pkgsLLVM and pkgsCross.<various-windows-flavors>. That's not cool.

[ghost]

Can we cross-compile from x86_64-linux to Darwin without Apple-proprietary tooling? If so, this should include some basic test of that.