Motivation for this change

The function value cannot be serialized so nix-env was mad. Turns out we can
just remove it like we do in lib/systems/inspect.nix.

@peti This is the fix. Sorry I didn't have it earlier. Hope the disruption wasn't too terrible.

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
  • other Linux distributions
• 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.