Follow #25229
At present, when the trigger event is `pull_request_target`, the `ref`
and `sha` of `ActionRun` are set according to the base branch of the
pull request. This makes it impossible for us to find the head branch of
the `ActionRun` directly. In this PR, the `ref` and `sha` will always be
set to the head branch and they will be changed to the base branch when
generating the task context.
Fix #25088
This PR adds the support for
[`pull_request_target`](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target)
workflow trigger. `pull_request_target` is similar to `pull_request`,
but the workflow triggered by the `pull_request_target` event runs in
the context of the base branch of the pull request rather than the head
branch. Since the workflow from the base is considered trusted, it can
access the secrets and doesn't need approvals to run.
