flatpak: can't run because schemas are not being found

(sm.puri.Calls:2): GLib-GIO-ERROR **: 07:22:47.406: Settings schema 'sm.puri.Calls' is not installed

I've checked gnome-calendar to see if anything special is being done for flatpak.

• What I'm doing: https://source.puri.sm/Librem5/calls/-/blob/master/data/meson.build#L71
• What calendar is doing: https://gitlab.gnome.org/GNOME/gnome-calendar/-/blob/master/data/meson.build#L53 https://gitlab.gnome.org/GNOME/gnome-calendar/-/blob/master/meson.build#L29

Probably I also need to take the prefix into account