Fix `ref` for workflows triggered by `pull_request_target` (#25743)

Fix `ref` for workflows triggered by `pull_request_target` (#25743)

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.