Only run coverage on push, not on pull_request

No need to run it twice, running it on the branch is enough,
no need to measure coverage on the merge commit too.

.github/workflows/ci.yml

@@ -25,6 +25,7 @@ jobs:
 
   coverage:
     runs-on: ubuntu-latest
+    if: ${{ github.event_name == 'push' }}
     steps:
       - name: Checkout sources
         uses: actions/checkout@v4