CMake: Fix configuration with dual-build
KDEInstallDirs6 includes ECMQueryQt.cmake, which tries to locate Qt5 version of qmake before we even have a chance to interfere.
Amends d60e88af
It fixes Qt 6 dev build with -DBUILD_QT5=OFF
for me, but breaks it with Qt5 ON. I guess, QUERY_EXECUTABLE
being a cache var does not play nicely with multi-version builds.