should work

.github/workflows/codingstyle.yml

@@ -1,5 +1,5 @@
 name: Check coding style
-on: [push]
+on: [push, pull_request]
 
 jobs:
   Building:
@@ -11,9 +11,10 @@ jobs:
       - name: Check coding style
         run: |
           cpplint --recursive --quiet --verbose=3 . 2> tmp
+          echo "CPP_LINT_LOG=$(cat tmp)" >> $GITHUB_ENV
       - name: Comment PR
         if: ${{ failure() && github.event_name == 'pull_request' }}
         uses: thollander/actions-comment-pull-request@master
         with:
-          message: ${{ github.event_name }}
+          message: ${{ env.CPP_LINT_LOG }}
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}