Tweak label creation logic on backport workflows

Signed-off-by: Peter Zhu <zhujiaxi@amazon.com>

.github/workflows/backport.yml

@@ -42,9 +42,11 @@ jobs:
 
       - name: Label new backport PR with backport-automerge label
         run: |
-          PR_BRANCH=backport/backport-<%= number %>-to-<%= base %>
+          PR_BRANCH=`echo ${{ github.event.label.name }}`
+          PR_BRANCH=`echo $PR_BRANCH | cut -d ' ' -f2`
+          PR_BRANCH=backport/backport-${{ github.event.pull_request.number }}-to-$PR_BRANCH
           PR_NUMBER=`gh pr list -R opensearch-project/documentation-website --json "number,headRefName" --state open | jq -r ".[] | select(.headRefName == \"$PR_BRANCH\") | .number"`
-          echo "Update Backport PR #$PR_NUMBER on branch $PR_BRANCH with backport-automerge label"
+          echo "Update Backport PR '#$PR_NUMBER' on branch '$PR_BRANCH' with 'backport-automerge' label"
           gh issue edit -R opensearch-project/documentation-website $PR_NUMBER --add-label backport-automerge
         env:
-          GH_TOKEN: ${{ steps.github_app_token.outputs.token }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}