On Windows 8.1, the one that is problematic to tell from Windows 10
(because the latter also exposes its version as 603 to the msiexec),
the registry value doesn't exist at
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion
but let's play safe and also check for "#6" value just in case.

Reference:
https://stackoverflow.com/questions/31072543/reliable-way-to-get-windows-version-from-registry

Thanks to Mitchell <blazer64@gmail.com> for the idea!

Change-Id: Ic907c4d992a7cb1d12e392686c19cd6fd6da3c7c
Reviewed-on: https://gerrit.libreoffice.org/65231Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
Tested-by: Mike Kaganski <mike.kaganski@collabora.com>