ci: fix checkout ref fallback in openapi job

Author: gildesmarais

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
       - uses: actions/checkout@v6
         with:
           token: ${{ secrets.RELEASE_PLEASE_TOKEN || github.token }}
-          ref: ${{ github.head_ref }}
+          ref: ${{ (github.event_name == 'pull_request' && startsWith(github.head_ref, 'release-please--')) && github.head_ref || github.sha }}
           fetch-depth: 0
 
       - uses: ruby/setup-ruby@v1