ci: check-out proper ref

Author: maxmoehl

.github/workflows/test-sample-apps.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v3
         with:
-          ref: "${{ github.event.pull_request.merge_commit_sha }}"
+          ref: "refs/pull/${{ github.event.number }}/merge"
       - name: Setup java/jdk
         uses: actions/setup-java@v3
         with: