Windows MSI installer: Fix upgrading from old MSI

When

1. Inkscape was installed from .MSI from Inkscape 1.3.x, and then
2. A newer Inkscape 1.4.x was built with CMake 3.29.{0...4} and then installed from .MSI

then the user had two half-broken installations instead of one new Inkscape version.

This is due to a CMake regression https://gitlab.kitware.com/cmake/cmake/-/issues/26029

The underlying change in CMake had originally intended to solve #1617 (closed) #1618 (closed) but was effectively reverted in CMake 3.29.5 because it created these half-broken installs.

Now we effectively re-open the two above bugs to avoid the new half-broken installation. If we wait some time until we use newer CMake we'll have the same behavior by default (but without explicit documentation).

Tested on Windows 10:

Old buggy behavior:

• Uninstall Inkscape
• Install Inkscape 1.3.2 MSI
• Install Inkscape dev e37552ac MSI 2024-06-25
• Two Inkscape uninstall entries shown.

New behavior with fixed upgrading but reversing the fix of #1617 (closed) #1618 (closed)

• Uninstall all old Inkscape installations (after the above bug, you will see two uninstall entries in the control panel!)
• Install Inkscape 1.3.2 MSI
• Install Inkscape from this merge request MSI
• One uninstall entry shown :-)
• But: Start menu entry is only visible for current user.