Build linux-musl targets

[stchris]

Waiting on axodotdev/cargo-dist#74 for this, because of openssl:

  thread 'main' panicked at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/openssl-sys-0.9.98/build/find_normal.rs:190:5:


  Could not find directory of OpenSSL installation, and this `-sys` crate cannot
  proceed without this knowledge. If OpenSSL is installed and this crate had
  trouble finding it,  you can set the `OPENSSL_DIR` environment variable for the
  compilation process.

  Make sure you also have the development packages of openssl installed.
  For example, `libssl-dev` on Ubuntu or `openssl-devel` on Fedora.

  If you're in a situation where you think the directory *should* be found
  automatically, please open a bug at https://github.com/sfackler/rust-openssl
  and include information about your system as well as this message.

  $HOST = x86_64-unknown-linux-gnu
  $TARGET = x86_64-unknown-linux-musl
  openssl-sys = 0.9.98