Do not create another auto PR when a job runs for PR (#573)

`.github/workflows/generate-code.yml` creates another PR when the
submitted PR has diff ... 😄
This change fixes it.

Author: Yang-33

.github/workflows/generate-code.yml

@@ -2,6 +2,8 @@ name: Generate code and open pull request
 
 on:
   push:
+    branches:
+      - 'master'
   pull_request:
   merge_group:
   workflow_dispatch: