document `nix-systems/default-linux` development requirement

[jleightcap]

I have received questions about failures like:

does not provide attribute 'pkgs.aarch64-darwin...'

For newcomers, it's probably best to clearly document that we do not support developing on systems other than x86_64-linux (see @lorenzleutgeb's justification: https://github.com/ngi-nix/ngipkgs/issues/24#issuecomment-2091429319)