VS: Add custom VCEnd labels only in C# projects

In commit dff98aa9ca (VS: add missing label in C# project-build events,
2021-12-15) the condition for adding our own `VCEnd` label was based on
the project being managed or not.  Since we support managed C++
projects, switch the condition to be based on whether the project is C#.

Issue: #21440
4 files changed