Add build support for linux-musl

[strophy]

Other protobuf repos contain several issues requesting builds (or at least testing) against musl libc on x86_64 and aarch64, e.g. under Alpine Linux.

• Musl compatible protoc-gen-js protobuf-javascript#153
• Ruby: Build on Alpine/musl protobuf#7040
• JS support on arm64 rvolosatovs/docker-protobuf#179

I'm fairly familiar with the build process, Alpine and GHA in general, and would be interested in contributing a PR. I noticed this CI is under heavy development at the moment - if someone could let me know if PRs are accepted and point to roughly where you would like it to be implemented, then I can give it a try.