Fix backport workflow (#315) (#318)

(cherry picked from commit 96435be)

Signed-off-by: Thomas Farr <tsfarr@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

.github/workflows/backport.yml

@@ -11,6 +11,15 @@ jobs:
     permissions:
       contents: write
       pull-requests: write
+    if: >
+      github.event.pull_request.merged
+      && (
+        github.event.action == 'closed'
+        || (
+          github.event.action == 'labeled'
+          && contains(github.event.label.name, 'backport')
+        )
+      )
     name: Backport
     steps:
       - name: GitHub App token
@@ -24,4 +33,4 @@ jobs:
         uses: VachaShah/backport@v2.2.0
         with:
           github_token: ${{ steps.github_app_token.outputs.token }}
-          branch_name: backport/backport-${{ github.event.number }}
+          head_template: backport/backport-<%= number %>-to-<%= base %>