fix: switch to statically linked `coreutils-*-linux-musl`

The `coreutils-*-linux-gnu` binaries have dynamic link to `glibc`.

Switching to the `musl` variants that are statically linked.

Leaving in the download targets for the `gnu` variants as we
_could_ register them with the `gnu` constraint from `toolchain_utils`
but there is not much point, because the `musl` variant will
always work.