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.

tests/integration/actions_trigger_test.go

@@ -138,7 +138,7 @@ func TestPullRequestTargetEvent(t *testing.T) {
 
 		// load and compare ActionRun
 		actionRun := unittest.AssertExistsAndLoadBean(t, &actions_model.ActionRun{RepoID: baseRepo.ID})
-		assert.Equal(t, addWorkflowToBaseResp.Commit.SHA, actionRun.CommitSHA)
+		assert.Equal(t, addFileToForkedResp.Commit.SHA, actionRun.CommitSHA)
 		assert.Equal(t, actions_module.GithubEventPullRequestTarget, actionRun.TriggerEvent)
 	})
 }