Specify repository info in openapi head checkout

.github/workflows/openapi.yml

@@ -13,7 +13,8 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v2
         with:
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.ref }}
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
       - name: Setup .NET Core
         uses: actions/setup-dotnet@v1
         with: