Update cabal-fmt.nix

Move from 9.2 to 9.6.

haskell.nix will drop support ghc < 9.6 in the future. And currently 9.2. doesn't even build on Darwin, as it requires 8.10 to bootstrap and that's broken, and will be removed from haskell.nix in the not too distant future.

Author: angerman

src/ext/cabal-fmt.nix

@@ -7,7 +7,7 @@ let
 
     version = "0.1.12";
 
-    compiler-nix-name = "ghc92";
+    compiler-nix-name = "ghc96";
 
     # Cabal is a lib library, so haskell.nix would normally use the one coming
     # from the compiler-nix-name (currently 3.2). However cabal-fmt depends on