Add comm cachix binary cache to the flake
This allows for commands such as nix develop to check the binary cache
in addition to nixpkgs' cache.nixos.org.
I've already uploaded the dev shell for x86_64-linux, x86_64-darwin (intel), and
aarch64-darwin (m1).
Cachix has a limited free tier for public caches, since we don't have anything
sensitive, I just created a comm cache.
If nix is not able to find a package in either cache, it will just fall back
to building from source (which is the current workflow).