Change push CI to run on workflow_run event
# What does this PR do?

The attempt in #17369 (to make commit history status checks less noisy) unfortunately has no effect.
After a discussion in [this comment](https://github.com/huggingface/transformers/pull/17369#issuecomment-1153846717), this PR changes push CI to be triggered by a `on: workflow_run` event.

Note the change only takes effect once this PR is merged into `main`, as mentioned in the doc. of [workflow_run](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_run).

The result would be like in [accelerate](https://github.com/huggingface/accelerate), where the jobs in `on-merge.yml` won't be shown, and the workflow run page look like [this](https://github.com/huggingface/accelerate/actions/workflows/on-merge.yml).
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger I merged this PR, you can check on the commit history page

[Change push CI to run on workflow_run event](https://github.com/huggingface/transformers/commits/main)

Hope you ❤️  it!<|||||>Amazing, thanks a lot!<|||||>I am sorry to bother you again ...