Add pixi builds for TSan + free-threading to aid downstream development.

[ngoldbaum]

This follows on from #142466, which was about ASan builds. Downstream is also doing TSan testing to some extent, mostly relying on the cpython_sanity images. See for example, this NumPy build: https://github.com/numpy/numpy/blob/06fb5b8a5dd0bb3ef83e24fa8b49f8ed643c8708/.github/workflows/compiler_sanitizers.yml#L81-L110.

This works for packages like NumPy with few dependencies but doesn't scale to projects with lots of dependencies. Ideally we'd be able to recursively build packages for the same reason people want to do this for ASan.

#142872 is a WIP implementation.

Linked PRs

• gh-143120: WIP pixi builds for free-threading and TSAN #142872