haskell.lib.buildStackProject is overridding shell hook and doesn't append
user-specified shellHook to it, resulting in user's shellHook
never executing.

I believe shellHook should be appended after addStackArgsHook has been executed, but I can see a case for being executed beforehand too so if that is preferred approach let me know and I'll fix it.

Motivation for this change

Ability to add shellHook commands to stack nix environments.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

Testing:

# shell.nix

{ pkgs ? import <nixpkgs> {}
}:

pkgs.haskell.lib.buildStackProject {
  inherit ghc;
  name = "stacktest";
  buildInputs = [
    pkgs-master.glpk
    pkgs-master.pcre
  ];
  
  shellHook = ''
    echo "Hello https://github.com/NixOS/nixpkgs!"
    '';

}

Result:

$ cd ~/dev/nixpkgs && git co upstream/master && cd - && NIX_PATH=nixpkgs=/path/to/nixpkgs nix-shell
HEAD is now at 9934f0bb51a AgdaStdlib: 0.15 -> 0.16 (#44550)

[nix-shell:~/dev/haskell-test]$ exit

$ cd ~/dev/nixpkgs && git co add-shellhook-support-to-buildstackproject && cd - && NIX_PATH=nixpkgs=/path/to/nixpkgs nix-shell
Previous HEAD position was 9934f0bb51a AgdaStdlib: 0.15 -> 0.16 (#44550)
Switched to branch 'add-shellhook-support-to-buildstackproject'
Hello https://github.com/NixOS/nixpkgs!