Automatically build and publish to flathub-beta from master

[skutter-de]

Is your feature request related to a problem? Please describe. I (like others) want to test new changes before a new release has been published.

Describe the solution you'd like I would suggest expanding the workflow to automatically publish each commit to flathub-beta to make testing easier.

[daudix]

Not sure about publishing to flathub beta, but downloading latest CI build would be nice