nixos/nixpkgs/read-only: fix cross‐compilation

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.

nixos/modules/misc/nixpkgs/read-only.nix

@@ -63,7 +63,7 @@ in
       # find mistaken definitions
       builtins.seq cfg.config builtins.seq cfg.overlays builtins.seq cfg.hostPlatform builtins.seq
         cfg.buildPlatform
-        cfg.pkgs;
+        cfg.pkgs.__splicedPackages;
     nixpkgs.config = cfg.pkgs.config;
     nixpkgs.overlays = cfg.pkgs.overlays;
     nixpkgs.hostPlatform = cfg.pkgs.stdenv.hostPlatform;