Run the CI on pull request.

This is needed to have the CI run on pull request created from of fork
respository.

We don't need to launch the CI on push :
- It will also be build on pull_request.
- The CI does check code only (no publication,...)

.github/workflows/check_pr.yml

@@ -1,6 +1,6 @@
-name: CI
+name: Check PR
 
-on: [push]
+on: [pull_request]
 
 jobs:
   Macos: