Adding necessary bits for cross-compiling to `aarch64-unknown-linux-gnu` to CI/CD

I got this working locally by:
```bash
$ rustup target add aarch64-unknown-linux-gnu
$ sudo apt-get install gcc-aarch64-linux-gnu g++-aarch64-linux-gnu
```
then adding to `.cargo/config.toml`:
```
[target.aarch64-unknown-linux-gnu]
linker = "aarch64-linux-gnu-gcc"
```
then I could
```bash
$ cargo build --target=aarch64-unknown-linux-gnu
```

Could we add this to the CI/CD pipeline?

Any thoughts from folks? Tagging in @AnotherDaniel, @sophokles73