enable nix-index and disable command-not-found

- Enable nix-index program to provide package search functionality
- Disable command-not-found as nix-index handles missing command suggestions instead
- Maintains consistency with recent nix configuration improvements

modules/nixos/nix/default.nix

@@ -52,6 +52,8 @@ in
     ];
 
     #systemd.services.nix-daemon.environment.TMPDIR = "/var/tmp";
+    programs.nix-index.enable = true;
+    programs.command-not-found.enable = false; # nix-index handles it instead
 
     nix =
       let