Don't try to install waylandtasksmodeltest into the build dir
It already exists in the build dir and since CMAKE_BINARY_DIR holds the full path it ends up trying to install into some ephemeral dir when packaging for a distro, e.g. "/var/tmp/paludis/build/kde-plasma-workspace-scm/work/build".
Instead just pass the proper path of the waylandtaskmodeltest to waylandtasksmodeltestwrapper.py, which admittedly assumes searching for a ECM version >= 5.38, but we already depend on a greater version.