cabal-cache marked as broken in nix

```
$ nix-shell '' -A haskellPackages.cabal-cache
error: Package ‘cabal-cache-1.0.0.12’ in /nix/store/jdv353aw8ip98chg7hlq8svsdw3iz3lf-nixpkgs-19.09pre184351.3ddbe024f73/nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix:44543 is marked as broken, refusing to evaluate.

a) For `nixos-rebuild` you can set
{ nixpkgs.config.allowBroken = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
{ allowBroken = true; }
to ~/.config/nixpkgs/config.nix.

(use '--show-trace' to show detailed location information)
```