Avoid Push CI failing to report due to many commits being merged
# What does this PR do?

We have increasing commits merged recently, and when it happens in a very period of short time (4 merges in ~ 1min yesterday), we get an error when using `actions/checkout@v2` in the `workflow_run` event for **Push CI**

```bash
fatal: reference is not a tree: 5f5e264a12956bd7cce47dcb422b80ed68e4c24e
```

So this PR increases the fetch depth to 20, and hopefully we are safe with this number 😆 
```
fetch-depth: 20
```
@sgugger Probably it's good for L to know this change - your call :-)<|||||>_The documentation is not available anymore as the PR was closed or merged._