VLC snap is built without using fully-updated repositories

For some reasons it seems that the VLC snap is actually built without including the xenial-updates repository, then various updates to the embedded libraries aren't picked up.

As side effect, for example the indicator isn't showing the proper icon (see https://forum.snapcraft.io/t/panel-icon-missing-telegram-snap/3922) and that's because VLC includes an old version of libappmenu-qt5, not the one available in the ubuntu xenial-updates repo.