tests: refactor to carry the package set as an argument

This way, the package set will be possible to pass without re-importing
all the time

nixos/lib/build-vms.nix

@@ -1,6 +1,4 @@
-{ system, minimal ? false, config ? {} }:
-
-let pkgs = import ../.. { inherit system config; }; in
+{ system, pkgs, minimal ? false, config ? {} }:
 
 with pkgs.lib;
 with import ../lib/qemu-flags.nix { inherit pkgs; };