nix develop broken in multiple ways

[turion]

• We have a dependency on https://github.com/TerrorJack/haskell-nix-utils/ in our flake.nix, which seems to be discontinued. It still uses GHC 8.6 which is long outdated and has to be compiled from source when doing nix develop. It seems it is only used in cabal-docspec, so maybe there is a different way to do this directly, see https://github.com/phadej/cabal-extras and https://serokell.io/blog/doctests-in-haskell. Also, functionality like this should be on CI so that it doesn't break.
• When removing this dependency, one gets:

  ❯ nix develop
warning: Git tree '/home/turion/haskell/monad-bayes' is dirty
warning: AWS error uploading 'nix-cache-info': Access Denied
error:
       … while calling the 'derivationStrict' builtin

         at //builtin/derivation.nix:9:12: (source not available)

       … while evaluating derivation 'nix-shell'
         whose name attribute is located at /nix/store/xzg2sw1gd8cbnhlijm6kmlf7ajgj3ikb-source/pkgs/stdenv/generic/make-derivation.nix:303:7

       … while evaluating attribute 'nativeBuildInputs' of derivation 'nix-shell'

         at /nix/store/xzg2sw1gd8cbnhlijm6kmlf7ajgj3ikb-source/pkgs/stdenv/generic/make-derivation.nix:347:7:

          346|       depsBuildBuild              = lib.elemAt (lib.elemAt dependencies 0) 0;
          347|       nativeBuildInputs           = lib.elemAt (lib.elemAt dependencies 0) 1;
             |       ^
          348|       depsBuildTarget             = lib.elemAt (lib.elemAt dependencies 0) 2;

       error: attribute 'mkJupyterlabFromPath' missing

       at /nix/store/kl5wh7cjx9a0kwqyrds4jy7i0galcyb1-source/flake.nix:56:18:

           55|     (
           56|       system: let
             |                  ^
           57|         inherit (nixpkgs) lib;

This is probably due to some recent API changes in https://github.com/tweag/jupyenv. These have to be tracked down and fixed. And nix develop should be run on CI as well.

[turion]

CC @idontgetoutmuch @reubenharry

[turion]

cabal-docspec unfortunately is hard to install in a sustainable way. See phadej/cabal-extras#135. The only way I can think of (apart of building from source) would be pinning a version from a github release as described in https://serokell.io/blog/doctests-in-haskell. The alternative is dropping cabal-docspec altogether, which would be sad, but might be necessary.

[turion]

It seems cabal-docspec was added as an experiment in #161, so I'm of a mind to remove it, since it was never tested on CI (and haskell-nix-utils seem unmaintained).