treewide: use attrs instead of list for types.loaOf options

nixos/modules/services/networking/ntp/chrony.nix

@@ -79,14 +79,10 @@ in
 
     environment.systemPackages = [ pkgs.chrony ];
 
-    users.groups = singleton
-      { name = "chrony";
-        gid = config.ids.gids.chrony;
-      };
+    users.groups.chrony.gid = config.ids.gids.chrony;
 
-    users.users = singleton
-      { name = "chrony";
-        uid = config.ids.uids.chrony;
+    users.users.chrony =
+      { uid = config.ids.uids.chrony;
         group = "chrony";
         description = "chrony daemon user";
         home = stateDir;

nixos/modules/services/networking/ntp/ntpd.nix

@@ -104,9 +104,8 @@ in
 
     systemd.services.systemd-timedated.environment = { SYSTEMD_TIMEDATED_NTP_SERVICES = "ntpd.service"; };
 
-    users.users = singleton
-      { name = ntpUser;
-        uid = config.ids.uids.ntp;
+    users.users.${ntpUser} =
+      { uid = config.ids.uids.ntp;
         description = "NTP daemon user";
         home = stateDir;
       };

nixos/modules/services/networking/ntp/openntpd.nix

@@ -60,8 +60,7 @@ in
 
     environment.etc."ntpd.conf".text = configFile;
 
-    users.users = singleton {
-      name = "ntp";
+    users.users.ntp = {
       uid = config.ids.uids.ntp;
       description = "OpenNTP daemon user";
       home = "/var/empty";