chore: use pixi for codegen environment in ci

[tokoko]

No description provided.

[benbellick]

I've never heard of pixi before so I may just not understand. But is there a reason we couldn't just use uv in CI, considering that we already have a uv.lock file checked in?

[tokoko]

it's just a faster alternative to conda, written in rust of course 😆 you can't configure system dependencies with uv (antlr, buf and protoc in this case). this PR only changes codegen check to use pixi which used whole devcontainer before. tests are still run with only uv. Although maybe it might make sense to port everything to pixi in the future as well for simplicity, not sure.

[giospada]

Can you update the CONTRIBUTING.md?

[tokoko]

done, also upgraded libprotobuf to latest in pixi as well

[nielspardon]

it feels to me for the time being (until substrait-io/substrait#925) one would always run codegen after updating the submodule. what do you think about linking them?

[nielspardon]

aka update-submodule triggers codegen but you can still do codegen independently

[tokoko]

turns out you can't link them like this update-submodule = [ "./update_submodule.sh", { task = "codegen" }], so I added another update-substrait task that runs both instead.

[nielspardon]

LGTM, much nicer approach with pixi