https://build.neon.kde.org/view/Snaps/job/bionic_release_extras_gcompris-qt_snap_amd64/
The issue is due because box2d is installed in /usr/lib/x86_64-linux-gnu/qt5/qml/Box2D.2.0/libBox2D.so and we look for it in /snap/kde-frameworks-5-core18-sdk/current/usr/lib/x86_64-linux-gnu/qt5/qml (which is found at https://cgit.kde.org/gcompris.git/tree/cmake/box2d.cmake#n7)
Qt seems to be installed in /snap/kde-frameworks-5-core18-sdk/current/usr/lib/x86_64-linux-gnu/libQt5Qml.so.5.12.3
Note that CMAKE_FIND_ROOT_PATH is also set to /snap/kde-frameworks-5-core18-sdk/current
Investigation is to know why box2d is not installed at same place or if it is normal that it is installed in the current place and how to get it.