Enable unsafe PR checkout in workflow

Author: jiaoshuntian

.github/workflows/pr-preview.yml

@@ -19,6 +19,7 @@ jobs:
           ref: ${{ github.event.pull_request.head.sha }}
           repository: ${{ github.event.pull_request.head.repo.full_name }}
           path: ivorysql_doc
+          allow-unsafe-pr-checkout: true
 
       - name: Fetch All Relevant Branches into Local Docs Repo
         working-directory: ./ivorysql_doc