Rely on nix to use latest ghc-9.6 point release

This isn't actively used; however, it's common to exchange the roles of
8.10.7 and the 9.6 release in the flake variants list. In lieu of
explicitly using 9.6.5, using nix' ghc96 alias for the latest point
release of 9.6 makes sense to avoid needing to churn the codebase at
each such release.

nix/haskell.nix

@@ -22,7 +22,7 @@ let
       name = "cardano-node";
       compiler-nix-name = lib.mkDefault "ghc8107";
       # extra-compilers
-      flake.variants = lib.genAttrs ["ghc964"] (x: {compiler-nix-name = x;});
+      flake.variants = lib.genAttrs ["ghc96"] (x: {compiler-nix-name = x;});
       cabalProjectLocal = ''
         repository cardano-haskell-packages-local
           url: file:${CHaP}