nixos/stalwart: improve code

* address review comments:
 https://github.com/NixOS/nixpkgs/pull/481815#discussion_r2719419333
 https://github.com/NixOS/nixpkgs/pull/481815#discussion_r2719421199

nixos/modules/services/mail/stalwart.nix

@@ -170,7 +170,7 @@ in
       users = {
         ${cfg.user} = {
           isSystemUser = true;
-          group = "${cfg.group}";
+          inherit (cfg) group;
         };
       };
     };
@@ -220,8 +220,8 @@ in
           StateDirectory = if useLegacyDefault then "stalwart-mail" else "stalwart";
 
           # Upstream uses "stalwart" as the username since 0.12.0
-          User = "${cfg.user}";
-          Group = "${cfg.group}";
+          User = cfg.user;
+          Group = cfg.group;
 
           # Bind standard privileged ports
           AmbientCapabilities = [ "CAP_NET_BIND_SERVICE" ];