fixed PR idempotency

Author: wtripp180901

.github/workflows/update-timestamps.yml

@@ -123,7 +123,8 @@ jobs:
         run: |
           set +e
           gh pr list --json headRefName --jq '.[].headRefName' | grep auto/bump-timestamps
-          echo "pr_exists=$?" >> "$GITHUB_OUTPUT"
+          PR_EXISTS=$?
+          echo "pr_exists=$PR_EXISTS" >> "$GITHUB_OUTPUT"
 
       - name: Create PR
         if: steps.pr-check.outputs.pr_exists