Add support for pixi venvs

[NewtonChutney]

Pixi, the modern alternative to conda (which has pip pre-installed), is written in Rust depending on uv as a lib, but not an installed package. So, unlike in conda's venvs, pixi's venvs aren't currently compatible with maturin:

[NewtonChutney]

For now, installing uv makes things work, but uv doesn't see the packages from conda, which pixi sees.

[messense]

I don't personally use pixi, so PR welcome to add support for it.

[NewtonChutney]

Hey there, I've been trying to debug the maturin bin.

I've been able to get this far...

1. I've installed uv in the pixi venv using pixi s to activate the venv and pixi a uv
2. When I run maturin develop --uv, I don't get any issue
3. I was able to debug the develop flow with the env vars copied from from an acitvated pixi shell
4. Next is to attempt adding functionality for detecting a pixi venv similar to uv.