ignore an externally set $GHC_ENVIRONMENT

Currently a bogus `$GHC_ENVIRONMENT` can make cabal fail. Cabal itself could trigger this issue if it's called inside a `cabal exec` (though this is probably a terrible idea in the first place). Local and default environments instead do not cause any interference.

Should we always pass `-package-env=-` to ghc, or set `$GHC_ENVIRONMENT` to `-`?

I think yes:

* Cabal manages the set of packages it passes to ghc on its own, so why should it be affected by external environments?
* I don't see any use for passing an environment to cabal
* it can cause inscrutable errors

on the other hand:

* resetting a variable that may heve been explicitly set by the user does not sound good