Checkout merge commit on pull_request_target event

Author: chuckwondo

.github/workflows/integration-test.yml

@@ -73,6 +73,8 @@ jobs:
 
       - name: Checkout source
         uses: actions/checkout@v4
+        with:
+          ref: "${{ github.event.pull_request.merge_commit_sha }}"
 
       - name: Install package with dependencies
         uses: ./.github/actions/install-pkg