Use intermediate env

.github/workflows/openapi.yml

@@ -45,10 +45,12 @@ jobs:
           repository: ${{ github.event.pull_request.head.repo.full_name }}
           fetch-depth: 0
       - name: Checkout common ancestor
+        env:
+          HEAD_REF: ${{ github.head_ref }}
         run: |
           git remote add upstream https://github.com/${{ github.event.pull_request.base.repo.full_name }}
           git -c protocol.version=2 fetch --prune --progress --no-recurse-submodules upstream +refs/heads/*:refs/remotes/upstream/* +refs/tags/*:refs/tags/*
-          ANCESTOR_REF=$(git merge-base upstream/${{ github.base_ref }} origin/${{ github.head_ref }})
+          ANCESTOR_REF=$(git merge-base upstream/${{ github.base_ref }} origin/${{ $HEAD_REF }})
           git checkout --progress --force $ANCESTOR_REF
       - name: Setup .NET
         uses: actions/setup-dotnet@3447fd6a9f9e57506b15f895c5b76d3b197dc7c2 # v3.2.0