Finally fully enable binary cache for nix-community and emacs-overlay.
No I don't have to build emacs from scratch for every master update :)
This was done by means of cachix, which is a means to upload built derivations
for others to share with. Nix-community has their own, and it makes using
overlays provided by nix-community that much better. This should shave off 20-25
min per system rebuild.
I am not sure if this will work as is after moving this config to new system,
so here are the steps to follow:
1. nix shell nixpkgs#cachix
2. sudo cachix use nix-community
3. stage/commit cachix.nix and cachix/ from /etc/nixos
4. done
Hopefully this will be enough in the future system move. If not, all the best to
future me.
