Fix GHC 9.12 devShell

Pin fourmolu and hlint to our usual versions, but only provide them in
the default devShell (9.6)

Author: sgillespie

flake.nix

@@ -188,16 +188,15 @@
             shell = {
               tools = {
                 cabal = "3.14.2.0";
-                fourmolu = "0.17.0.0";
-
-                # We'd prefer 3.8, but it won't work on all compilers we support. Instead
-                # we'll let solver sort it out.
-                hlint = "latest";
 
                 haskell-language-server = {
                   src = nixpkgs.haskell-nix.sources."hls-2.11";
                 };
               } // lib.optionalAttrs (config.compiler-nix-name == "ghc967") {
+                # These versions work with GHC 9.6, but not with 9.10 and 9.12
+                fourmolu = "0.17.0.0";
+                hlint = "3.8";
+
                 weeder = "latest";
               };