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
is initialized to ProgramFiles/ProgramFiles64 by default.
32 jobs for !5016 with cpack_wix_install_scope in 72 minutes and 3 seconds (queued for 1 second)
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