ci: Add a flatpak job

Build and run the test-suite inside the flatpak enviroment, so that we
actually test the recommended deployment and development mechanism[0].

As a bonus, also produce a flatpak bundle as a build artifact that can
be downloaded to easily try out a proposed change before it is merged.

[0] https://gitlab.gnome.org/GNOME/Initiatives/wikis/DevOps-with-Flatpak

#61
4 jobs for flatpak-ci-job in 6 minutes and 35 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #71138
build-polari

00:02:13

passed #71139
flatpak

00:05:53

 
  Review
passed #71140
review

00:00:20

passed #71141
manual
stop_review

00:00:19