release(-lib).nix: optionally allow building unfree packages

[bjornfor]

I'd like to create a personal nixpkgs channel containing some unfree
packages. This adds 'allowUnfree' parameter so that more of the release
infrastructure can be reused.

The default is still to not build unfree packages, so for nixpkgs this
is no functional change.

Motivation for this change

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 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.

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[mention-bot]

@bjornfor, thanks for your PR! By analyzing the history of the files in this pull request, we identified @edolstra, @domenkozar and @rbvermaa to be potential reviewers.

[bennofs]

Perhaps we should just allow passing in a config arg? This would cover more usecases than just this one.

[bjornfor]

Good point.

Should we go further? We could pass an arbitrary attrset, so that e.g. overlays can be specified too?

...
, nixpkgsArgs ? { config = { allowUnfree = false; inHydra = true; }; }