Add benchmark of pixi with uv

[olivier-lacroix]

Summary

Adds Pixi as one of the tool benchmarked against uv.

• Pixi can generate multi-platform lock files. here, lock-files are limited to the platform the benchmark is executed on
• Pixi can install packages from conda and/or pypi. here, packages are installed from pypi

Still in draft as generating lock-files currently fails due to https://github.com/prefix-dev/pixi/issues/817