Visual Studio Build Tools should not use floating version shortlink

[lbussell]

#1271 updates the VS Build Tools to the latest version by downloading them from https://aka.ms/vs/stable/vs_BuildTools.exe. The previous URL was https://aka.ms/vs/17/vs_BuildTools.exe, but there is no such shortlink (yet) for VS 18. The URL should be updated from stable to a fixed-version URL when possible to avoid future unanticipated breaking changes.