[Python test] Fix gcf permission denied error (#37684)

`grpc_distribtests_gcp_python` test is [failing with permission error](https://fusion2.corp.google.com/ci;ids=1930537984/kokoro/prod:grpc%2Fcore%2Fmaster%2Flinux%2Fgrpc_distribtests_gcp_python/activity/2c6d57dd-f504-4834-9fd6-845db47ede01/log):
```
ERROR: (gcloud.functions.deploy) ResponseError: status=[403], code=[Ok], message=[Permission 'run.services.setIamPolicy' denied on resource 'projects/grpc-testing/locations/us-central1/services/grpc-gcf-distribtest-3cd3cc11-1a8f-4b88-9c3b-220f0bdf9fdc' (or resource may not exist).]
```

In the logs, looks like we're creating gen2 functions by default:
```
As of this Cloud SDK release, new functions will be deployed as 2nd gen  functions by default. This is equivalent to currently deploying new  with the --gen2 flag. Existing 1st gen functions will not be impacted and will continue to deploy as 1st gen functions.
```

Since we don't need functions provided by gen2 functions and use `--no-gen2` fixed the permission issue, we're adding `--no-gen2` flag to our test.

### Test:
* Passed manually run: http://sponge/e52ffb74-5a12-4e67-9f95-4638390ef57c

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Closes #37684

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37684 from XuanWang-Amos:fix_gcf 26588c0cc3
PiperOrigin-RevId: 675703636

test/distrib/gcf/python/run_single.sh

@@ -44,7 +44,8 @@ function cleanup() {
 trap cleanup SIGINT SIGTERM EXIT
 
 # Deploy
-DEPLOY_OUTPUT=$(gcloud functions deploy "${FUNCTION_NAME}" --entry-point test_publish --runtime "${RUNTIME}" --trigger-http --allow-unauthenticated)
+
+DEPLOY_OUTPUT=$(gcloud functions deploy "${FUNCTION_NAME}" --entry-point test_publish --runtime "${RUNTIME}" --trigger-http --allow-unauthenticated --no-gen2)
 HTTP_URL=$(echo "${DEPLOY_OUTPUT}" | grep "url: " | awk '{print $2;}')
 
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