CPack/WiX: Set "InstallScope" to "perMachine"
This avoids a number of issues (start menu entries / desktop icons only installed for the current user, deployment tools failing due to the installation only being registered for the current user, etc.)
It's also more consistent with the fact that CPACK_WIX_SKIP_PROGRAM_FOLDER is initialized to ProgramFiles/ProgramFiles64 by default.
Incidentally it's also what CMake's own installer is doing (which happens to use a custom template instead of the default one).
Fixes a number of downstream bugs: