generic-builder.nix: Update doHaddock

By default, `ghcjs` haddocks are precluded from being built. I see no reason to disallow building haddocks on projects built with `ghc` and `ghcjs` (someone can correct me here). `HaLVM` currently does not support `haddock` since it's a Stage 1 `GHC`. https://github.com/GaloisInc/HaLVM/blob/master/src/misc/build.mk.in#L20

Currently, building `haddocks` for `ghcjs` projects requires altering the derivation, which doesn't allow users to take advantage of the cache. This change will relieve that.