Add support for other systems

[stv0g]

Currently, nix-build proxy seems to only support x86_64-linux:

https://github.com/polygon/nix-buildproxy/blob/c26d73992ddae96812501b5ae1cc45037d8b10be/flake.nix#L8C15-L8C28

We should try to use something like flake-utils' eachDefaultSystem to make it usable with other systems.

[polygon]

I mainly left those out because I can only test on x86_64-linux easily. Anything ending in -linux is probably goind to be alright, but I'm already less sure about -darwin.

Which systems would you like to see added, and could you test this on them?

[stv0g]

I can add support and test aarch64-Darwin