Pass legacy root node ProjectTreeFlags

This fixes a regression from the dependency tree rewrite. Implementations of the legacy extension point `IProjectDependenciesSubTreeProvider.CreateRootDependencyNode` return an `IDependencyModel` holding various values to be applied to the group's node. One such value is `IDependencyModel.Flags` which should be applied to the group node. This change fixes that.

src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Tree/Dependencies/Legacy/LegacyDependencySubscriber.cs

@@ -80,7 +80,7 @@ public ProviderState(IProjectDependenciesSubTreeProvider provider)
                 normalGroupIcon: rootNode.Icon.ToProjectSystemType(),
                 warningGroupIcon: rootNode.UnresolvedIcon.ToProjectSystemType(),
                 errorGroupIcon: rootNode.UnresolvedIcon.ToProjectSystemType(),
-                groupNodeFlags: ProjectTreeFlags.Empty);
+                groupNodeFlags: rootNode.Flags);
 
             _snapshot = ImmutableDictionary<DependencyGroupType, ImmutableArray<IDependency>>.Empty
                 .Add(_dependencyType, ImmutableArray<IDependency>.Empty);