Fix build issue

Author: Jack251970

Flow.Launcher.Core/Plugin/PluginManager.cs

@@ -576,7 +576,7 @@ public static async Task<bool> UpdatePluginAsync(PluginMetadata existingVersion,
             var uninstallSuccess = await UninstallPluginAsync(existingVersion, removePluginFromSettings: false, removePluginSettings: false, checkModified: false);
             if (!uninstallSuccess) return false;
 
-            _modifiedPlugins.Add(existingVersion.ID);
+            ModifiedPlugins.Add(existingVersion.ID);
             return true;
         }