Since commit 6051a49c (Visual Studio: Add Android support, 2020-06-12,
v3.19.0-rc1~619^2) we run MSBuild to build a sample project to detect
the sysroot.  Previously we relied on `CMAKE_VS_MSBUILD_COMMAND` being
available.  That required commit d5b5c192 (cmGlobalGenerator:
FindMakeProgram() before CMakeDetermineSystem, 2020-06-15,
v3.19.0-rc1~619^2~3) to make it available early enough.  However, that
ordering broke `CMAKE_GENERATOR_INSTANCE` so we need to prepare to
revert it.  Use `cmake_host_system_information` to get the location of
MSBuild under a VS generator instead.