VS: C# multi-target projects overwrite their own results during build

!7713 (merged) introduced an issue with C# multi-target projects which made the build process overwrite its own results. By forcing AppendTargetFrameworkToOutputPath to false all build outputs for all framework targets are written into the same directory. As a solution AppendTargetFrameworkToOutputPath should only be forced to false if the project is not a multi-target project.