Update explorer_preview.yml

.github/workflows/explorer_preview.yml

@@ -10,6 +10,8 @@ jobs:
     if: ${{ github.event.workflow_run.conclusion == 'success' }}
     steps:
       - uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
       - uses: amondnet/vercel-action@v20
         with:
           vercel-token: ${{ secrets.VERCEL_TOKEN }} # Required