Musl support in CI

Are there any available hosts that support musl? If so, how would I go about configuring a `cc_toolchain` for it, otherwise, I suppose this would be a feature request to enable CI tests for musl (related to https://github.com/bazelbuild/rules_rust/pull/829).

Start by investigating whether CI hosts support musl and how a cc_toolchain would be configured for it. Use the related rules_rust pull request as context, then determine whether a usable configuration or CI test coverage can be established; done requires a confirmed host and an agreed path forward.

Written by the indexing model from the issue text.