Specify correct branch name to Wrangler action

The Cloudflare Wrangler action doesn't detect the branch name for
`pull_request` triggers, so we have to specify it explicitly.

.github/workflows/preview.yml

@@ -42,7 +42,7 @@ jobs:
         with:
           apiToken: ${{ secrets.API_TOKEN }}
           accountId: ${{ secrets.ACCOUNT_ID }}
-          command: pages deploy dist --project-name=${{ vars.PROJECT_NAME }}
+          command: pages deploy dist --project-name=${{ vars.PROJECT_NAME }} --branch ${{ github.head_ref || github.ref }} --commit-hash ${GITHUB_SHA}
           workingDirectory: frontend
       - name: Upload artifact
         uses: actions/upload-artifact@v4