test: fix pull request target action ref (#27649)

* test: pull request target action

* fix checkout pr ref

.github/workflows/ui.yml

@@ -11,6 +11,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: checkout
+        if: github.event_name == 'pull_request_target'
+        uses: actions/checkout@master
+        with:
+          ref: refs/pull/${{ github.event.pull_request.number }}/merge
+
+      - name: checkout
+        if: github.event_name == 'push'
         uses: actions/checkout@master
 
       - name: install