Fix playlists leaderboard provider not being thread safe

[bdach]

Should close #34222.

I haven't exactly managed to reproduce the issue myself but I'm relatively confident in the imagined mode of failure here. See

osu/osu.Game/Screens/OnlinePlay/Playlists/PlaylistsPlayer.cs

Lines 25 to 37 in 861a7e1

	[Cached(typeof(IGameplayLeaderboardProvider))]
	private readonly PlaylistsGameplayLeaderboardProvider leaderboardProvider;
	protected override UserActivity InitialActivity => new UserActivity.InPlaylistGame(Beatmap.Value.BeatmapInfo, Ruleset.Value);
	public PlaylistsPlayer(Room room, PlaylistItem playlistItem)
	: base(room, playlistItem, new PlayerConfiguration
	{
	ShowLeaderboard = true,
	})
	{
	leaderboardProvider = new PlaylistsGameplayLeaderboardProvider(room, playlistItem);
	}

osu/osu.Game/Screens/OnlinePlay/Playlists/PlaylistsPlayer.cs

Line 53 in 861a7e1

LoadComponentAsync(leaderboardProvider, AddInternal);

Also comes with a side of (slightly) reduced number of collection change events fired.