nixos/lib/testing.nix: make 'config' a free variable

cc @edolstra

nixos/lib/build-vms.nix

···
-{ system, minimal ? false }:
+{ system, minimal ? false, config ? {} }:
 
-let pkgs = import ../.. { config = {}; inherit system; }; in
+let pkgs = import ../.. { inherit system config; }; in
 
 with pkgs.lib;
 with import ../lib/qemu-flags.nix;

nixos/lib/testing.nix

···
-{ system, minimal ? false }:
+{ system, minimal ? false, config ? {} }:
 
-with import ./build-vms.nix { inherit system minimal; };
+with import ./build-vms.nix { inherit system minimal config; };
 with pkgs;
 
 rec {