[Serve] Fix standalone3 tests (ray-project#34100) (ray-project#34135)

environment is not inherited in Windows with subprocess, we need to explicitly inject env variables.

The reason we don't find it before is because the test is inside the standalone2.py, which is ignored for windows. 

windows passed.
```
//python/ray/serve:test_standalone3                                      PASSED in 207.6s
```

python/ray/serve/tests/test_standalone3.py

@@ -375,7 +375,9 @@ def predict(self, inp):
 assert ray.get(handle.predict.remote(1)) == 1
     """
 
-    run_string_as_driver(script, env={SYNC_HANDLE_IN_DAG_FEATURE_FLAG_ENV_KEY: "1"})
+    run_string_as_driver(
+        script, dict(os.environ, **{SYNC_HANDLE_IN_DAG_FEATURE_FLAG_ENV_KEY: "1"})
+    )
 
 
 if __name__ == "__main__":