Fix/cleanup our pixi tasks

[jleibs]

What

• Resolves: pixi run py-test fails with undefined reference to memfd_create'` #6852

Most notably, this introduces a new py environment to mirror the cpp environment.

py-build is now scoped to that environment, with py-build-examples being a dedicated task scoped to the examples environment.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!
• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

To run all checks from main, comment on the PR with @rerun-bot full-check.

[github-actions]

Deployed docs

Commit	Link
60382b3	https://landing-diov4aimy-rerun.vercel.app/docs

[jleibs]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/9892318022

[jleibs]

@rerun-bot full-check

[github-actions]

Started a full build: https://github.com/rerun-io/rerun/actions/runs/9894749098

[Wumpf]

Great docs! I tried a few commands locally (Mac) and it seems to work fine.

[emilk]

This causes problems when running docs/snippets/compare_snippet_output.py, since it build the python wheel to a different environment than the one running the snippets.

We either need to add -e py to all python invocations in compare_snippet_output.py (but I think that will break CI, which calls compare_snippet_output.py with -e wheel-test), or call py-build-common from compare_snippet_output.py instead of py-build. But then I wonder - what is the py-build command for?

Setting the environment inside the environment like this seems like an inherently bad idea since it means the environment the user asked for will be ignored