ci: build wheels for musllinux

[MrMino]

Summary

Allow CIBW to build wheels with -musllinux tags.

Alpine CIs could greatly benefit from having libcst available.

Closes #962

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[zsol]

Thanks!

[MrMino]

@zsol I wasn't sure if this is it, hence the "Draft:" in the name - sorry for not using the Draft feature, I brain farted and thought this works similarly to GitLab.

It looks like it's still failing on rustup: https://github.com/Instagram/LibCST/actions/runs/11878027367/job/33098223245

[MrMino]

It looks like the i686-unknown-linux-musl is only tier-2 support in rustup (source) hence the missing installer. @zsol If I understand this correctly, given the aforementioned missing support in Rust, it is ok to skip the builds for that plat+arch?

Edit: my rustup target list actually shows i686-unknown-linux-musl as supported, not sure what's going on here. Maybe it's just a thing with the PyPA images? A missing package somewhere? 🤔

Edit 2: Apparently, in the world of Rust, there is a distinction between Tier 2 support with and without host tools, which I wasn't aware of. So we can cross-compile for i686+linux+musl, but we should not expect to be able to get a rustup/cargo/rustc that itself works on this triplet.

Given how niche triplet this is (apparently it's something mostly used by people who run iSH app on IPad to get a Linux shell there), I'd say: skip it. I'll PR that one in a sec.