We pretty often get at least one job failed because of failure to pull
the musl git repo. Switch this to the unofficial mirror [1] which should
be more reliable.
Link: https://github.com/kraj/musl [1]
The submodule was causing issues in rust-lang/rust, so eliminiate it
here. `build-musl` is also removed from `libm-test`'s default features
so the crate doesn't need to be built by default.