Add Flatpak build CI

[TheEvilSkeleton]

Licensing

• I confirm that this is either my code or was released under the terms of a GPLv3-or-later compatible license. Also I agree to release it here under the terms of the GPLv3-or-later.

Description

This MR will help us test Tubefeeder flatpak without needing to manually build it. GitHub will publish artifacts that we can then download and install locally and test. This also enables us to use GNOME Builder to build it locally and launch the app without installing it.

Linked Issue

There weren't any discussion prior to opening this MR, so I completely understand if this gets closed.

[TheEvilSkeleton]

@Schmiddiii I noticed you use emojis for git commit messages. GitHub has a feature that lets you squash commits, which also lets you rewrite commit messages:

You can use that feature to add your preferred emoji.

[Schmiddiii]

Could we have the de.schmidhuberj.tubefeeder.json and player_downloader.diff not in the root-folder?

And I know that I can squash commits, but I do not really care about the emoji-commit-messages any more (I switched to another style for my newer projects). And this would probably un-verify your commits so I just left them.

[TheEvilSkeleton]

Is under build-aux good for you? I placed everything there since it's what other GNOME apps do.

[TheEvilSkeleton]

Uhh, I'm not sure why it needs network access out of nowhere...

[Schmiddiii]

It has to download Cargo packages from crates.io. Do you know how to fix this?

[TheEvilSkeleton]

I think I know how to fix it, but I'm really confused with why it was working completely fine locally, but not with CI.