allowing compiler.update for MSBuildToolchain

[memsharded]

Changelog: Bugfix: MSBuildToolchain explicitly adds the specific toolset .props file when compiler.update is defined, otherwise, activating vcvars is not enough.
Docs: Omit

Close #19125

[jcar87]

I'm assuming this essentially mirrors the MSBuild instructions here: https://devblogs.microsoft.com/cppblog/side-by-side-minor-version-msvc-toolsets-in-visual-studio-2019/

and that the usual caution applies w.r.t ensuring this happens before <Import Project="$(VCTargetsPath)\Microsoft.Cpp.Default.props" />??

[memsharded]

Yes, mostly, but instead of doing the copy and referring to it by relative path to the project, it does exactly the same as CMake, defines the full path to the original location of the .props file.

and that the usual caution applies w.r.t ensuring this happens before ??

yes, exactly, in our templates and tests this always happens, but it is true that it is not explicitly documented. I think it is a good idea to add it to the docs explicitly.