Nix binary cache for flake

[anna328p]

This project is a Nix flake. There's a service called Cachix that lets you set up a binary cache to store CI builds, so that Nix users can download binaries for the master branch instead of having to build it locally.

Cachix has very good GitHub Actions integration and is free for open source projects. Having this set up would be helpful for Nix users and take nearly no effort to maintain.

This needs to be done by the maintainer because it requires secrets to be generated and set for the repository.