test: fix file description passing to cloud-setup mock service

The pass_fds file descriptor is *after* the dup2. Always 3.

src/tests/client/test-client.py

@@ -2172,7 +2172,7 @@ class TestNmCloudSetup(TestNmClient):
                 [sys.executable, service_path],
                 stdin=subprocess.PIPE,
                 env=env,
-                pass_fds=(s.fileno(),),
+                pass_fds=(3,),
                 preexec_fn=pass_socket,
             )