workflows/backport: cancel concurrent runs (#414800)

Author: wolfgangwalther

.github/workflows/backport.yml

@@ -9,6 +9,10 @@ on:
   pull_request_target:
     types: [closed, labeled]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event_name }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 permissions:
   contents: read
   issues: write