Use the runtime ghc libdir for ghc-exactprint

[jneira]

• Using the env var hack added in master
• This uses the recently releases apply-refact-0.9.0.0
• This would close hlint seeking unavailable file /opt/ghc/8.8.4/lib/ghc-8.8.4/settings #591 and Applying hlint hint on Windows fails with "could not detect mingw toolchain" #615
• Using the env var could fix other code using ghc-exactprint
  • Retrie fails on Windows with could not detect mingw toolchain #471 for the retrie plugin
  • Formatting with Brittany does nothing #412 for brittany
  • But not sure how to generalize its use, being a hack

[jneira]

But not sure how to generalize its use, being a hack

Maybe it should be set in the entry of the hls executable (Main.hs) but theoretically you could have a multicradle using different ghc's (so with different libdirs). So we have to set the mutable state in the client code.
Also, it would be nice if somebody could suggest a cheap way to synchonize the environment setEnv, unsetEnv 🙂

[alanz]

Gets the job done

[jneira]

I've applied it to brittany to close #412