Checkout action needs explicit sha for pull request target

.github/workflows/integration.yaml

@@ -22,6 +22,8 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
 
       - name: Setup Python
         uses: actions/setup-python@v2