We should reduce our haskell overlay to the smallest size possible. It should ideally only contain overrides necessary to fix build we then can upstream and reap the benefits of with the next channel bump. Changing often used packages in the overlay causes an effective haskellPackages rebuild with the added problem that parallel building has been disabled upstream since it harms reproducibility.
The main culprit currently is random 1.2.0 which we need to override globally since otherwise propagation breaks xanthous's configure phase.
A good first step would thus be to port xanthous to random 1.1 (again).