* Support building Qt using CMake 3.28+
CMake 3.28 dropped support for deriving installation prefix(es) based on
PATH environment variable on Windows. Since all the other built packages
already pass necessary prefix path(s) explicitly via CMAKE_PREFIX_PATH
and it works for them, do the same for Qt 6 as well. Building Qt 5 isn't
affected as it doesn't use CMake.
* Fix environment variable name in `windows` build job
Using wrong name resulted in empty `CMAKE_PREFIX_PATH` CMake setting
value, which showed the effects of CMake 3.28 changes yet again.
Bring back old configuration that was using Qt 5 and split Qt build in
two to support both Qt 5 and Qt 6. Last Qt 5 builds were using 5.14.2,
bump that to latest 5.15.8.
Qt 6 only supports Windows 10+, so adjust the system version check
accordingly. Not bumping Qt 6 version (although 6.4.2 is available)
while we're still in patch release mode.
Building a whole new package is not nice since Qt client is only one
part of it, the rest doesn't require newer OS version. Nevertheless,
it's a quick and easy fix.
Mike Gelfand