Use PR branch for preview (#225)

Author: Ernesto Castellotti

.github/workflows/preview-pr.yaml

@@ -13,6 +13,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
         with: