nixos/tests: Make pure

Just had @fabianhauser at the #ZurichZHF hackathon run into this, causing the
tests to fail evaluation due to some system impurity

nixos/tests/make-test-python.nix

···
 f: {
   system ? builtins.currentSystem,
-  pkgs ? import ../.. { inherit system; },
+  pkgs ? import ../.. { inherit system; config = {}; overlays = []; },
   ...
 } @ args: