nixos/test-driver: Use Python's callPackage

The original code meant `nix-shell` didn't pass the relevant arguments to `default.nix`:

> error: evaluation aborted with the following error message:
> 'lib.customisation.callPackageWith: Function called without required
> argument "buildPythonApplication" at
> […]/nixos/lib/test-driver/default.nix:4'

Co-Authored-By: Jacek Galowicz <jacek@galowicz.de>

nixos/lib/test-driver/shell.nix

@@ -1,4 +1,4 @@
 {
   pkgs ? import ../../.. { },
 }:
-pkgs.callPackage ./default.nix { }
+pkgs.python3Packages.callPackage ./default.nix { }