nixos/tests/unifi: inherit allowUnfree into test node

mongodb is unfree, so the test won't run without this. we already allow
unfree to grab the unifi packages, so it's not even a big change.

nixos/tests/unifi.nix

···
     };
 
     nodes.server = {
+      nixpkgs.config = config;
+
       services.unifi = {
         enable = true;
         unifiPackage = unifi;