Tweak label creation logic on backport workflows

Signed-off-by: Peter Zhu <[email protected]>

.github/workflows/backport.yml

@@ -42,9 +42,7 @@ jobs:
 
       - name: Label new backport PR with backport-automerge label
         run: |
-          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-$PR_BRANCH
+          PR_BRANCH=backport/backport-${{ github.event.pull_request.number }}-to-`echo ${{ github.event.label.name }} | cut -d ' ' -f2`
           PR_NUMBER=`gh pr list -R opensearch-project/documentation-website --json "number,headRefName" --state open | jq -r ".[] | select(.headRefName == \"$PR_BRANCH\") | .number"`
           echo "Update Backport PR '#$PR_NUMBER' on branch '$PR_BRANCH' with 'backport-automerge' label"
           gh issue edit -R opensearch-project/documentation-website $PR_NUMBER --add-label backport-automerge