haskell-generic-builder: update and jailbreak cabal files before patchPhase is run

The updated and jailbroken Cabal file must be in place before the
'patchPhase' hook is run, otherwise we cannot use that hook to patch the
Cabal file! Resolves https://github.com/NixOS/nixpkgs/issues/5922.

As an added bonus, this change makes the "jailbreakPhase" obsolete.