- Add a ghc environment including prebuilt dependencies to the
  nix-shell. Get rid of the ad hoc cabal cache and all dependencies
  are now downloaded from the nixos binary cache.
- Make gen_ci.hs a cabal package with HLS integration, to make future
  hacking of gen_ci.hs easier.
- Fix some ghc/hlint warnings after I got HLS to work.
- For the lint-ci-config job, do a shallow clone to save a few minutes
  of unnecessary git checkout time.

(cherry picked from commit 93b9bbc1)