uv speedup

[g4challenge]

Pipelines is already using uv in the dockerfile.

Yet in main.py and start.sh some "old" python pip is still in existence.

For pipelines being easier, and also potentially safer - i would wish and recommend to adapt main.py and start.sh for using uv by default.

[SamuelBortolinAfliant]

I'd love to see uv used by default into the pipelines.