ihaskell/wrapper.nix: unify ihaskellEnv and ihaskell

[vaibhavsagar]

Motivation for this change

Having both ihaskell and ihaskellEnv is redundant, and might even
cause problems if the two differ somehow.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-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/)
• Fits CONTRIBUTING.md.

---

[vaibhavsagar]

I've implemented this here with no issues.

[vaibhavsagar]

Are any changes required?

[vaibhavsagar]

Any obstacles to having this merged?

[adisbladis]

cc @edwtjo (original author) @peti (haskell maintainer)

[vaibhavsagar]

Thanks!