Partially fix#34710
The bug described in #34710 can be divided into two parts: `push.paths`
and `pull_request.paths`. This PR fixes the issue related to
`pull_request.paths`. The root cause is that the check for whether the
workflow can be triggered happens **before** updating the PR’s merge
base. This causes the file-change detection to use the old merge base.
Therefore, we need to update the merge base first and then check whether
the workflow can be triggered.
f4e38e6367