Make Slack CI reporting stronger
# What does this PR do?

Make Slack CI reporting stronger.

The most important change in this PR is to **use a token when we need to grab some GitHub workflow/jobs information** using api call, like
```python
https://api.github.com/repos/huggingface/transformers/actions/runs
```
to get all job links.

**This could avoid reaching the rate limit in CI runs and keep the CI reporting working.**

Such error occurred once on  2023/02/24, see [this run](https://github.com/huggingface/transformers/actions/runs/4258755021/jobs/7424107028). The log shows `Unknown error, could not fetch links. 'jobs'`, but the underlying reason (I strongly believe) is the rate limit is reached, and the api call returns 2 keys `message` and `documentation` without the key `jobs`.

The other changes are just to make things better too.

_The documentation is not available anymore as the PR was closed or merged._