make-test-python: disallow aliases

When importing Nixpkgs within Nixpkgs, we should not consider aliases
to ensure we don't rely on them internally.

There are probably more places that need to be converted.

nixos/tests/make-test-python.nix

@@ -1,6 +1,6 @@
 f: {
   system ? builtins.currentSystem,
-  pkgs ? import ../.. { inherit system; config = {}; },
+  pkgs ? import ../.. { inherit system; },
   ...
 } @ args: