[R] Revert "Update preview.yml"

This reverts commit f8f7e1a.

.github/workflows/preview.yml

@@ -9,6 +9,9 @@ on:
       pr_number:
         description: 'Pull Request number to deploy preview for'
         required: true
+      pr_branch:
+        description: 'The branch name to deploy'
+        required: true
 
 jobs:
   build:
@@ -30,15 +33,17 @@ jobs:
         run: |
           if [ "${{ github.event_name }}" == "workflow_dispatch" ]; then
             echo "PR_NUMBER=${{ github.event.inputs.pr_number }}" >> $GITHUB_ENV
+            echo "PR_BRANCH=${{ github.event.inputs.pr_branch }}" >> $GITHUB_ENV
           else
             echo "PR_NUMBER=${{ github.event.pull_request.number }}" >> $GITHUB_ENV
+            echo "PR_BRANCH=${{ github.event.pull_request.head.ref }}" >> $GITHUB_ENV
           fi
 
       # Now check out the PR’s head branch (whether from a PR event or supplied manually)
       - name: Checkout PR branch
         uses: actions/checkout@v4
         with:
-          ref: "refs/pull/${{ env.PR_NUMBER }}/merge"
+          ref: ${{ env.PR_BRANCH }}
 
       - name: Setup Node.js
         uses: actions/setup-node@v4