[Nix] Add comm binary cache to flake

Summary:
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).

Test Plan:
Follow instructions added in D4817, to allow your user to modify substitutor behavior

Then, from a new machine (or run nix-collect-garbage with all gcroots removed):

nix develop
# assert that nodejs-16.3.0 came from cachix

Reviewers: atul, varun, ashoat

Reviewed By: atul, ashoat

Subscribers: tomek, adrian, abosh

Differential Revision: https://phab.comm.dev/D4816