[CI] Fix test_collective_rpc (#17858)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

Author: russellb

tests/entrypoints/llm/test_collective_rpc.py

@@ -10,7 +10,7 @@
 @pytest.mark.parametrize("tp_size", [1, 2])
 @pytest.mark.parametrize("backend", ["mp", "ray"])
 @create_new_process_for_each_test()
-def test_collective_rpc(tp_size, backend):
+def test_collective_rpc(tp_size, backend, monkeypatch):
     if tp_size == 1 and backend == "ray":
         pytest.skip("Skip duplicate test case")
     if tp_size == 1:
@@ -21,6 +21,7 @@ def test_collective_rpc(tp_size, backend):
     def echo_rank(self):
         return self.rank
 
+    monkeypatch.setenv("VLLM_ALLOW_INSECURE_SERIALIZATION", "1")
     llm = LLM(model="meta-llama/Llama-3.2-1B-Instruct",
               enforce_eager=True,
               load_format="dummy",