VS: Use MSBuild matching toolset host architecture
VS 2017 and VS 2019 provide
amd64/MSBuild.exe variants next to their
MSBuild.exe tools. When the 64-bit host toolchain is selected (e.g. via
host=x64), select the 64-bit MSBuild too.
Similar to !1317 (merged), this requires delaying finding the native build tool until after the toolset selection.