enable architecture specific downloads

[jnettlet]

This is an early feature enhancement request to start thinking about adding architecture specific downloads for airshipper. We are very close to enabling Aarch64 support on Veloren and it would be nice to use airshipper in order to download and run the latest builds. At least Fedora already has an Aarch64 build of Airshipper packaged, however it downloads x86_64 binaries.

I see that Airshipper just builds a default URL based on the cfg(target_os), so most likely the easiest solution would also use cfg(target_arch), and then build the URL https://download.veloren.net/latest/linux/aarch64 or something similar.