Incorrect CMAKE_TOOLCHAIN_FILE check

While preparing (or trying at least) qtwebengine 6.2 alpha, the build fails with :

[ 106s] CMake Error at /usr/share/cmake/Modules/CMakeDetermineSystem.cmake:129 (message):
[ 106s] Could not find toolchain file:
[ 106s] /home/abuild/rpmbuild/BUILD/qtwebengine-everywhere-src-6.2.0-alpha/build/qtbase/lib/cmake/Qt6/qt.toolchain.cmake
[ 106s] Call Stack (most recent call first):
[ 106s] CMakeLists.txt:14 (project)

This raises two issues:

• why would qt.toolchain.cmake need to be check for simple builds
• this file is only certain to exist in this location if QT_SUPERBUILD is TRUE or QT_WILL_INSTALL is FALSE (set in QtBuildInternalsExtra.cmake) and INSTALL_LIBDIR wasn't customized