Use sha hash instead of branch name for OpenAPI workflow head reference (#7340)

.github/workflows/openapi.yml

@@ -14,7 +14,7 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v2
         with:
-          ref: ${{ github.event.pull_request.head.ref }}
+          ref: ${{ github.event.pull_request.head.sha }}
           repository: ${{ github.event.pull_request.head.repo.full_name }}
       - name: Setup .NET Core
         uses: actions/setup-dotnet@v1