Microsoft.TemplateEngine.Authoring.Tasks not incremental

[KirillOsenkov]

ValidateTemplates and LocalizeTemplates tasks and their targets don't appear to be incremental.

When building Aspire.ProjectTemplates, it takes 960 ms and 340 ms every time, even if the templates don't change.

https://github.com/dotnet/aspire/blob/b2136486902bf253f7487cea1bb6e2d4781fb40b/src/Aspire.ProjectTemplates/Aspire.ProjectTemplates.csproj#L14