Fix confusing working directory in Push CI
# What does this PR do?

⚠️ ~~Let me launch a tiny run to make sure before merge~~ 🙏

Previously, the job `setup` (to prepare the tests to run) in push CI ran on `ubuntu-latest` runners, and has to clone `transformers` with `actions/checkout@v2`.

In #19054, it is changed to use the docker image (and run on our runner). This is no particular reason for it, just to make the sequence of jobs looks more linear.

But I forgot to change the working directory in #19054. The job still run without failure (as the 2 `transformers` directory exist at the same time), but it looks somehow confusing. This PR is to clean this.
_The documentation is not available anymore as the PR was closed or merged._