workflows/comment: fix workflow syntax

Github doesn't evaluate expressions unless they're inside ${{ }}

See: https://github.com/mpv-player/mpv/actions?query=is%3Afailure+event%3Aworkflow_run

.github/workflows/comment.yml

@@ -7,8 +7,7 @@ on:
 
 jobs:
   pr_comment:
-    if: github.event.workflow_run.event == 'pull_request' &&
-        github.event.workflow_run.conclusion == 'success'
+    if: ${{ github.event.workflow_run.event == 'pull_request' && github.event.workflow_run.conclusion == 'success' }}
     runs-on: ubuntu-latest
     steps:
       - env: