Gitlab CI: build flatpak nightlies
Per the DevOps with Flatpak initiative https://gitlab.gnome.org/GNOME/Initiatives/wikis/DevOps-with-Flatpak Fixes #41 (closed)
Per the DevOps with Flatpak initiative https://gitlab.gnome.org/GNOME/Initiatives/wikis/DevOps-with-Flatpak Fixes #41 (closed)