Add multi-arch Flatpak CI builds with ARM64 support

[SolsticeSpectrum]

Fixes #751

Testing is requried, it automatically creates releases using the softprops/action-gh-release github action
I tested on postmarketOS, altough I had to run these commands first to fix GPG issues and install gnome 47

flatpak remote-delete flathub
flatpak remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo

flatpak install flathub org.gnome.Platform//47

[Diegovsky]

I'm a bit on the fence here regarding merging this as I have no expertise with CI and don't have arm devices to test it out. Are you willing to become a contributor and help out with this? If so, I wouldn't mind.

[SolsticeSpectrum]

I'm a bit on the fence here regarding merging this as I have no expertise with CI and don't have arm devices to test it out. Are you willing to become a contributor and help out with this? If so, I wouldn't mind.

sure, I have no problem with that. I have been using the build for quite some time now and have no issues.

[Diegovsky]

This is really cool! Development has shifted into https://github.com/Diegovsky/riff for now. I have disabled CI because I don't have experience with it and it was running every time despite being broken, but I am willing to accept arm support when we have CI up again!