Disable all Haskell builds for Darwin on Hydra.

Hydra generates a GHC closure for Darwin that for no apparent reason
contains an ancient, broken Haddock binary -- probably because of an
impurity in the build system. That bug makes those GHC binaries
unusable: <https://github.com/NixOS/nixpkgs/issues/2689>.
Patch 7a20d76 failed when the Haskell expression lacked a meta attribute. This commit fixes that issue.