ci(actions): fix issue comment condition

Author: jrolfs

.github/workflows/build.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     if:
       ${{ github.event_name == 'pull_request' || github.event_name == 'push' ||
-      contains(github.event.issue_comment, 'rebuild') }}
+      contains(github.event.comment.body, 'rebuild') }}
 
     steps:
       - name: Get Yarn cache path