CScript must be run as 64-bit regardless of the MSBuild bitness. Otherwise it won't find 64-bit Git installations.
However the "Sysnative" redirector is not available for 64-bit processes. So a fix is needed when 64-bit MSBuild is run.
The "ProgramFiles(x86)" Macro is only set for 64-bit, otherwise it is empty. Therefore it can be used as condition to check whether the current MSBuild process is 32 or 64-bit.
Build Events are run in an 32 bit environment, therefore both program files environment strings resolve to the x86 program files folder on 64 Bit systems. If Git is 64 bit and installed into the x64 program files it can't be found by the script.
Julian Löhr