CPack/WiX: Restore default (missing) InstallScope from 3.28 and lower

In commit 60661f6770 (CPack/WiX: Make InstallScope configurable,
2023-11-07, v3.29.0-rc1~413^2) the `CPACK_WIX_INSTALL_SCOPE` default was
set to `perMachine`.  While installers created with `perMachine` have
better behavior on their own than installers created without any
`InstallScope`, they do not cleanly convert existing installations on
updates.

Fixes: #26029