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

Changed files
+1 -1
nixos
+1 -1
nixos/tests/make-test-python.nix
···
f: {
system ? builtins.currentSystem,
-
pkgs ? import ../.. { inherit system; },
+
pkgs ? import ../.. { inherit system; config = {}; overlays = []; },
...
} @ args: