VS: Regression of CMAKE_GENERATOR_INSTANCE in toolchain file
The CMAKE_GENERATOR_INSTANCE documentation states that it may be initialized in a toolchain file.
Since !4898 (merged) and !5529 (merged), the Visual Studio generators select the location of
MSBuild.exe before loading the toolchain file (which is done in
CMakeDetermineSystem). A given
MSBuild will use its own VS instance by default, so the effect of this ordering is that
CMAKE_GENERATOR_INSTANCE does not work when initialized by a toolchain file.