ci: change uv sync to uv pip install

[gsprochette]

Description

uv sync is messing up the CI when bumping the transformers compatibility to 5+. By resolving the whole environment at the same time, it ends up with 4.57, whereas for most extras 5+ is compatible.
This PR changes uv sync to uv pip install and slightly cleans up the installation process.

Related Issue

Fixes #(issue number)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactor (no functional change)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Testing

• I added or updated tests covering my changes
• Existing tests pass locally (uv run pytest -m "cpu and not slow")

For full setup and testing instructions, see the Contributing Guide.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code, especially for agent-assisted changes
• I updated the documentation where necessary

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[gsprochette]

Note that the CI still installs transformers==4.57.6 because we still have a pin on transformers, This PR is in preparation of removing the pin, to be able to install transformers>=5 in #685
The torch version has changed to 2.12, which is fine because we're only using cpu in this workflow, otherwise we would need the extra-index from pytorch to get a non-blackwell build...

[begumcig]

Let's gooooooooooooo