Fix ruleset selector not updating to the first ruleset item until after LoadComplete()

This fixes the whole issue behind `Ruleset.Value` being null, by
updating `Current` on BDL rather than waiting for the base logic which
executes at `LoadComplete`.

This seems like something that should happen at the base `TabControl` class itself, by switching `Current` right after the first added tab item, rather than doing it on `LoadComplete`, but I'm not sure about changing framework logic outright, so fixing this locally until it occurs on other places.

osu.Game/Rulesets/RulesetSelector.cs

@@ -1,6 +1,7 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using System.Linq;
 using osu.Framework.Graphics.UserInterface;
 using osu.Framework.Allocation;
 
@@ -18,6 +19,12 @@ namespace osu.Game.Rulesets
         {
             foreach (var r in Rulesets.AvailableRulesets)
                 AddItem(r);
+
+            // This is supposed to be an implicit process in the base class, but the problem is that it happens in LoadComplete.
+            // That can become an issue with overlays that require access to the initial ruleset value
+            // before the ruleset selectors reached a LoadComplete state.
+            // (e.g. displaying RankingsOverlay for the first time).
+            Current.Value = Items.First();
         }
     }
 }