Missing Cocoa system dependency on macOS

[purefn]

Builds are failing on macOS with nixpkgs-unstable since the last nixpkgs update in 008e8cc. They're failing with

       error: The Nixpkgs package set does not contain the package: Cocoa (system dependency).
       You may need to augment the system package mapping in haskell.nix so that it can be found.

[hamishmack]

This should have been fixed by #2376