fix(nixos): conditionally apply onFailure units

- Wrap `config.onFailure` in `mkIf cfg.enable` to ensure units are conditionally applied based on the service's `enable` configuration.
- Prevents unnecessary configuration of failure units when the service is disabled.

modules/nixos/services/systemd-email-notify/default.nix

@@ -77,7 +77,7 @@ in
       type =
         with types;
         attrsOf (submodule {
-          config.onFailure = onFailureUnits;
+          config.onFailure = mkIf cfg.enable onFailureUnits;
         });
     };
   };