Cross-compiling Nu can be a little tricky due to dependencies. This PR
makes it easy to use [`cross-rs`](https://github.com/cross-rs/cross), a
popular tool for cross-compiling Rust code using Docker:
```bash
cross build --target aarch64-unknown-linux-musl --release
```
I find this useful for compiling ARM binaries from x64. Easy to add more
target triples later as needed.
