Fix issue with doc build sometimes failing silently in GH workflows

[regisss]

What does this PR do?

This PR aims at avoiding silent errors when building the documentation in a Github workflow (see here for an example).

It seems that

defaults:
  run:
    shell: bash -l {0}

that is used in the main documentation build workflow makes some errors silent. Removing it since it is not necessary for building the doc.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

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

[fxmarty]

LGTM