please make use of flathub-beta channel for test builds

[bam80]

Could we please make an unofficial test build of https://gitlab.com/Remmina/Remmina/-/merge_requests/2507?
For that, flathub-beta repository exists.
Could we make use of it?

The way this works is that each GitHub repository builds the master branch for the stable repository, which will have the flatpak branch name “stable”, and then the beta git branch will build into the beta repository with the flatpak branch “beta”.

https://blogs.gnome.org/alexl/2019/02/19/changes-in-flathub-land/

So essentially we need to create "beta" branch here which points to master or some commit needed to test.