196f6a812b
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.