Add a (cachix) binary cache

This would be nice for consumers so they don't have to build the package from scratch.

You can add this to flake.nix and consumers will be prompted to accept it (or via `--accept-flake-config`). Example: https://github.com/willruggiano/neovim.drv/blob/main/flake.nix#L36-L45

You can then add a github CI workflow that pushes to cachix on push/release/whenever you want.