chore: Disable command-not-found by default

Unfortunately command-not-found only works for channels and doesn't have
first-class support for flakes yet, and nix-index takes forever to build
the database on slower machines, so I'd rather just disable this by
default.

modules/system.nix

@@ -218,6 +218,8 @@ in
       (pass.withExtensions (ext: with ext; [ pass-otp ]))
     ];
 
+    programs.command-not-found.enable = false;
+
     environment.defaultPackages = [ ];
   };
 }