Mark test_task_workflow_trigger_skipped as flaky (apache#41115)

The suite of tests in TestWorkflowTrigger regarding the async io
seems pretty flaky - it keeps on failing randomly and we keep
on adding more tests as flaky - this is the next one.

We likely should address it more generically and implement better
harness for those tests - this is captured in apache#41114

tests/triggers/test_external_task.py

@@ -137,6 +137,7 @@ async def test_task_workflow_trigger_fail_count_eq_0(self, mock_get_count):
         with pytest.raises(StopAsyncIteration):
             await gen.__anext__()
 
+    @pytest.mark.flaky(reruns=5)
     @mock.patch("airflow.triggers.external_task._get_count")
     @pytest.mark.asyncio
     async def test_task_workflow_trigger_skipped(self, mock_get_count):