Apply suggestions from code review

Use proper arguments to apply labels from original PR to backport PR.
Improve target branch detection.

Co-authored-by: AbsurdlyLongUsername <22662897+absurdlylongusername@users.noreply.github.com>

.github/workflows/backport-pr.yml

@@ -15,7 +15,7 @@ jobs:
     # everybody to trigger backports and create branches in our repository.
     if: >
         github.event.issue.pull_request &&
-        startsWith(github.event.comment.body, '/backport') &&
+        startsWith(github.event.comment.body, '/backport ') &&
         (
         github.event.comment.author_association == 'OWNER' ||
         github.event.comment.author_association == 'COLLABORATOR' ||
@@ -28,23 +28,19 @@ jobs:
         run: |
           set -euo pipefail
           body="${{ github.event.comment.body }}"
-          labels_json='${{ toJSON(github.event.pull_request.labels || github.event.issue.labels) }}'
-          target=$(echo "$body" | sed -n 's/.*\/backport[[:space:]]\+\([^[:space:]]\+\).*/\1/p')
-
-          if [ -z "$target" ]; then
-            echo "No backport target found in comment." >&2
+          
+          line=${body%%$'\n'*} # Get the first line
+          if [[ $line =~ ^/backport[[:space:]]+([^[:space:]]+) ]]; then
+            echo "BACKPORT_TARGET=${BASH_REMATCH[1]}" >> "$GITHUB_ENV"
+          else
+            echo "Usage: /backport <target-branch>" >&2
             exit 1
           fi
-          echo "BACKPORT_TARGET=$target" >> $GITHUB_ENV
 
-          labels=$(echo "$labels_json" | jq -r 'if . == null then "" else map(.name) | join(",") end')
-          if [ -z "$labels" ]; then
-            echo "BACKPORT_PR_LABELS=backport" >> $GITHUB_ENV
-          else
-            echo "BACKPORT_PR_LABELS=backport,$labels" >> $GITHUB_ENV
-          fi
       - name: Create backport pull request
         uses: korthout/backport-action@v4
         with:
-          add_labels: ${{ env.BACKPORT_PR_LABELS }}
+          add_labels: 'backport'
+          copy_labels_pattern: '.*'
+          label_pattern: ''
           target_branches: ${{ env.BACKPORT_TARGET }}