fix: benchmark trigger

Author: b-ma

.github/workflows/benchmark.yaml

@@ -17,7 +17,7 @@ env:
 jobs:
   benchmark:
     runs-on: ubuntu-latest
-    if: ${{ github.event.issue.pull_request }} && contains(github.event.comment.body, '/bench')
+    if: contains(github.event.comment.body, '/bench')
 
     steps:
       # the workflow is triggered from main branch