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/lib/testing-python.nix

@@ -130,7 +130,13 @@ rec {
               {
                 virtualisation.qemu.package = qemu_pkg;
               }
-            );
+            ) ++ [(
+              {
+                # Ensure we do not use aliases. Ideally this is only set
+                # when the test framework is used by Nixpkgs NixOS tests.
+                nixpkgs.config.allowAliases = false;
+              }
+            )];
           };
 
           # FIXME: get this pkg from the module system

nixos/tests/make-test-python.nix

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