nixosTests.xfce: silence a deprecation warning

trace: warning: Module argument `nodes.machine.config` is deprecated. Use `nodes.machine` instead.

nixos/tests/xfce.nix

···
     };
 
   testScript = { nodes, ... }: let
-    user = nodes.machine.config.users.users.alice;
+    user = nodes.machine.users.users.alice;
   in ''
       machine.wait_for_x()
       machine.wait_for_file("${user.home}/.Xauthority")