Remove cachix

[rossabaker]

We are trying to wind down the Typelevel Cachix instance. (See: typelevel/typelevel-nix#210). This is the other place we publish to it.

Without it, CI will be a little slower, as it has to rebuild the Jekyll environment on each pull request. Developers who use the Nix shell will have to do the same, though the Nix store will cache it for them until the dependencies change.