haskellPackages: inconsistent nix-shell behavior

[deepfire]

Issue description

It is harder to use nix-shell to reproduce Haskell build problems, because of the following:

nix-shell appears to have inconsistent behavior, which is contrary to what is described on its manpage:

The command nix-shell will build the dependencies of the specified derivation, but
not the derivation itself. It will then start an interactive shell in which all
environment variables defined by the derivation path have been set to their
corresponding values, and the script $stdenv/setup has been sourced. This is useful
for reproducing the environment of a derivation for development.

Specifically -- there appears to be no easy way to have both of the following:

1. build inputs available
2. *Phase variables set

Granted, the Haskell version of the mandate goes somewhat beyond the letter (but not the spirit) of the above description, since we also expect the build inputs not just being assigned to environment variables, but also registered within the GHC package data base.

Steps to reproduce

$ nix-shell -A 'pkgs.haskellPackages.haskell-gi'     -E 'import <nixpkgs> {}' --run 'echo buildPhase=$buildPhase; ghc-pkg list | wc -l'
buildPhase=runHook preBuild ./Setup build runHook postBuild
30
$ nix-shell -A 'pkgs.haskellPackages.haskell-gi.env' -E 'import <nixpkgs> {}' --run 'echo buildPhase=$buildPhase; ghc-pkg list | wc -l'
buildPhase=
73

cc @peti

[deepfire]

Just realised that #36393 was merged recently!