git-annex.cabal: Add back custom-setup stanza, so cabal new-build works.

Seems I had all the work in past commits to make this build, at least on
linux. I'm actually surprised it does, without a unix dep, Utility.Env
still builds ok somehow despite using System.Posix.Env.

This commit was sponsored by Fernando Jimenez on Patreon.

Git/CurrentRepo.hs

@@ -12,6 +12,7 @@ import Git.Types
 import Git.Construct
 import qualified Git.Config
 import Utility.Env
+import Utility.Env.Set
 
 {- Gets the current git repository.
  -