Add update manager which performs no update action

This is to be used in cases where updates are handled by an external
means. See
https://github.com/flathub/flathub/pull/2619#issuecomment-969731305 for
initial usage.

osu.Desktop/OsuGameDesktop.cs

@@ -93,6 +93,11 @@ namespace osu.Desktop
 
         protected override UpdateManager CreateUpdateManager()
         {
+            string packageManaged = Environment.GetEnvironmentVariable("OSU_EXTERNAL_UPDATE_PROVIDER");
+
+            if (!string.IsNullOrEmpty(packageManaged))
+                return new NoActionUpdateManager();
+
             switch (RuntimeInfo.OS)
             {
                 case RuntimeInfo.Platform.Windows: