track latest master in Flatpak repo

[bam80]

I remember there was demand last year about such possibility:
flathub-infra/flatpak-builder-lint#144 (comment)

I recalled about it as we at KDE created something similar, but on Gitlab infrastructure.
See example for Koko project here:
https://invent.kde.org/graphics/koko/-/blob/master/.gitlab-ci.yml
and here:
https://invent.kde.org/sysadmin/ci-utilities/-/blob/master/gitlab-templates/flatpak.yml

I wounder if we could do something similar.

Ref:

Automation & Systematization
Koko now produces Flatpak bundles with every commit and Merge Request change via its CI system (Tobias Fella, link)

https://pointieststick.com/2023/08/25/this-week-in-kde-tap-to-click-by-default/