publishing to Flathub so that Buzztrax works regardless of OS updates

[trusktr]

The Arch Linux installer in the AUR fails, presumable due to updates in dependencies.

I think we could mitigate this sort of breakage by offering a way for people to install Buzztrax via Flatpak, which bundles all system dependencies into a self-contained package.

We can automate the flatpaks with GitHub actions.