If INSTALL_LIBEXECDIR was set to something != "libexec",
qt-configure-module contained the wrong path to qt-cmake.
Fixes: QTBUG-128137
Pick-to: 6.5 6.7 6.8
Change-Id: Ic095ff8cb804bbdd72e238e75ac867b7cc3bd478
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>