do not run size limit job outside of PRs

Currently, this step emits the
"Error: No PR found. Only pull_request workflows are supported."
message when run on the master branch commits (after the merge),
so the change prevents it from being run there.

.github/workflows/ci-frontend.yml

@@ -150,6 +150,7 @@ jobs:
   size-limit:
     name: Size limit
     runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
     env:
       CI_JOB_NUMBER: 1
     steps: