`python3` instead of `python` in Push CI setup job
# What does this PR do?

The `setup` job in push CI uses image `transformers-all-latest-gpu-push-ci`, which should use `python3` instead of `python`.
(I forgot this detail when working on #19054)

Currently, the setup job failed, and no test to run.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Actually, `python` is not found. I think we need to tweak a bit if we want to use `python` as `python3`. Do you want me to work on the docker image for  this purpose?

```bash
/__w/_temp/5f33851e-ac7e-4126-b3d7-88092ae0b56d.sh: 2: python: not found
```<|||||>No, don't worry. I was just complaining about the action env, not your PR :-) 