CI for the dev shell

[rossabaker]

It would be nice if this had CI for the dev shell. It would look a lot like the one in typelevel-nix, and catch issues like #221.

[armanbilge]

If it can be boiled down to a static file (or almost something so simple) maybe we can configure sbt-typelevel to spit it out.

[rossabaker]

The only parameters I can foresee are the cachix instance name and the name of the environment variable, and there's not really any reason to vary the latter.