Add flatpak packages for nightly build to flathub-beta

[MithicSpirit]

Though there is a nightly release on github, this does not include a flatpak version. If possible, it would be nice if these were also added to flathub-beta (rather than regular flathub so as to not cause issues with the "stable" package).

[nukeop]

It should be pretty easy to do, there are already packages in https://github.com/NuclearPlayer/ that use github actions to update AUR and snapcraft. The flatpak repo could be triggered the same way (by the release action in this repo).

[UltraBlackLinux]

Yep, I would love to see this as well

[nukeop]

I started working on this