gha: don't check commits on push

This check (for commit subject being short) only make sense for pull
requests, and it fails on pushes to master (i.e. on merges).

Make sure it's only run for pull requests.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/validate.yml

@@ -85,6 +85,8 @@ jobs:
 
   commit:
     runs-on: ubuntu-20.04
+    # Only check commits on pull requests.
+    if: github.event_name == 'pull_request'
     steps:
       - name: get pr commits
         id: 'get-pr-commits'