fix default ghc version selection

Author: Providence Salumu

flake.nix

@@ -203,7 +203,7 @@
             ghc94 = ghc94Config.tweakHpkgs (pkgs.hlsHpkgs "ghc94");
             ghc96 = ghc96Config.tweakHpkgs (pkgs.hlsHpkgs "ghc96");
           };
-          in { default = cases."${ghcVersion}"; } // cases;
+        in { default = cases."${builtins.substring 0 (builtins.sub (builtins.stringLength ghcVersion) 1) ghcVersion}"; } // cases;
 
         ghc90 = supportedGHCs.ghc90;
         ghc92 = supportedGHCs.ghc92;