QNX toolchain files are incomplete and cause host includes to be found

The QNX provisioning contains an archive with cmake toolchain files that are used in the CI when building Qt. https://codereview.qt-project.org/c/qt/qt5/+/324032/24/coin/provisioning/common/linux/qnx_710.sh#72

These files are incomplete, in the sense that they do not restrict the lookup of host includes, libraries and packages.

This cause qtbase integrations to pick up host Vulkan headers and broke the build https://testresults.qt.io/coin/integration/qt/qtbase/tasks/1625054006
and had to be circumvented here https://codereview.qt-project.org/c/qt/qtbase/+/357515/3/src/gui/configure.cmake#754

The toolchain files should set the various CMake CMAKE_FIND_ROOT_PATH variables to restrict host lookups, similar to how it's done in the yocto-provided cmake toolchain files.