nixpkgs release: Fix Darwin-only jobs

Currently the logic of generating nixpkgs Hydra jobs is to walk through
the pkgs evaluated for system = "x86_64-linux", collect any derivations
and their meta.platforms values. However, that doesn't work for
packages whose meta.platforms doesn't include x86_64-linux, as just
evaluating their meta attribute raises an error so they get skipped
completely.

As a less-intrusive fix (i.e. anything than rewriting the current package
enumeration logic), allow passing `config.allowUnsupportedSystem = true`
to permit evaluating packages regardless of their platform and use that
in the package listing phase.

Fixes #25200