Proposal: Don't read environment files by default

[Matthew Pickering]

Hi all,

Environment files have caused a large amount of pain for users because
they are read by default by GHC.

For example: https://github.com/haskell/cabal/issues/4542

Cabal developers have indicated that they are not going to stop
generating them by default despite the overwhelming user pressure.

Therefore I propose that users should opt-in to using environment
files by having to explicitly pass a flag to enable the search
behavior.

This will provide a much better user experience overall and will stop
tooling having to isolate itself from their existence.

Cheers,

Matt