use semgrep.yml for postmerge checks only

.github/workflows/semgrep.yml

@@ -1,24 +1,17 @@
 on:
   workflow_dispatch: {}
-  pull_request: {}
   push:
     branches:
     - main
-    - master
-    paths:
-    - .github/workflows/semgrep.yml
-  schedule:
-  # random HH:MM to avoid a load spike on GitHub Actions at 00:00
-  - cron: 5 4 * * *
 name: Semgrep
 jobs:
   semgrep:
     name: semgrep/ci
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     env:
       SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
     container:
-      image: returntocorp/semgrep
+      image: semgrep/semgrep
     steps:
     - uses: actions/checkout@v3
     - run: semgrep ci