Revert "Change push CI to run on workflow_run event"
Reverts huggingface/transformers#17692

Really sorry, but the `notification_service.py` has error

```
Traceback (most recent call last):
  File "utils/notification_service.py", line 766, in <module>
    ci_author = ci_details["author"]["login"]
KeyError: 'author'
```

as the GH event is no longer coupled with a commit, and we lose some information about commit/author.
I need to change more things.

(On my own repo for testing, there is no `notification_service.py`. And this change of `workflow_run` can only be verified when being merged to `main`)
_The documentation is not available anymore as the PR was closed or merged._<|||||>merged to avoid further test failures