This matches the definition of `_module.args.pkgs` in the standard
Nixpkgs module, and fixes a bunch of build issues caused by use of
`nativeBuildInputs = [ pkgs.foo ];` throughout NixOS.
A fallback could be added in case we expect people might be passing
in package sets without `__splicedPackages`, although that seems like
it would be inherently broken to me.
196f6a812b