br: git action optimization

[fengou1]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Pull Request
1, Create
2, Update,
3, Change
4, Merge

2. What did you expect to see? (Required)

Any action (create, update, change...) on a BR folder will trigger compatibility test
Git Action has limited resource, BR PR cost 30+ minutes which impact other repo Action

3. What did you see instead (Required)

Only Merge Action will trigger the compatibility test

4. What is your TiDB version? (Required)