fix: reuse already-found user in ParticipantsList instead of redundant lookup

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/c574fc49-f3fa-4386-9706-f05472318292

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot

osu.Game/Screens/OnlinePlay/Multiplayer/Participants/ParticipantsList.cs

@@ -64,7 +64,7 @@ private void updateState()
                 for (byte i = 0; i < slotUserIds.Length; ++i)
                 {
                     var user = slotUserIds[i] != null ? client.Room.Users.SingleOrDefault(u => u.UserID == slotUserIds[i]) : null;
-                    var participant = user == null ? Slot.Empty(i) : Slot.FromUser(client.Room.Users.Single(u => u.UserID == slotUserIds[i]));
+                    var participant = user == null ? Slot.Empty(i) : Slot.FromUser(user);
 
                     if (i >= slots.Count)
                         slots.Add(participant);