haskell-hackage-db: add override to fix build with GHC 8.4.1

Cc: @deepfire

pkgs/development/haskell-modules/configuration-ghc-8.4.x.nix

···
   # https://github.com/bitemyapp/esqueleto/issues/77
   esqueleto = markBrokenVersion "2.5.3" super.esqueleto;
 
+  # Older versions don't compile.
+  hackage-db = super.hackage-db_2_0_1;
+
 }