Update the sandbox set up
With the introduction of bubblewrap, linux-user-chroot has been completely superceded.
Unfortunately, GNOME Continuous still depends on it, and porting that project to bubblewrap is not a lightweight effort.
In the meantime, let's try and keep the delta between linux-user-chroot and the sandbox created by Flatpak to a minimum, to avoid weird build issues that do not manifest when running the build under Flatpak or BuildStream.
Edited by Emmanuele Bassi