nix - update flake.nix to better support cross-system eval

[colemickens]

Describe your feature request

Hi! My workflow for Nix is that I evaluate locally on my machine to create derivations, then I build those derivations on a powerful remote machine with a matching system architecture. In this way I can specify homogenous config for my machines regardless of architecture and coordinate eval/builds from my development machine without needing to think too hard about it.

Adding helix to my configurations threw a wrench in things because nix-cargo-integration was not properly evaluating cross-system -- when I would nix build --eval-store auto ... .#something-aarch64 on my x86_64 machine, the eval would fail. This sort of soft-blocks my deployments, and more-over is something I was hoping would get fixed because I want to see more awesome Rust projects adding good native Nix support.

Per issues on other repos, it seems like this is now fixed, but will require some changes to the flake. I'm not sure if I'll be able to get to this soon, so I wanted to file this.

See here for more details: yusdacra/nix-cargo-integration#58 (comment)

[colemickens]

cc: @the-mikedavis