Build Flatpak from GitHub CI

[AFCMS]

Problem

The Flathub version of Minetest is the recommended installation method under Linux (the first item in bold on the download page), but build isn't tested in CI.

Being able to download a Flatpak bundle from the build artifacts would also be beneficial for people who want to test early versions of Minetest without compiling it. (especially since Flatpaks can be installed at the user level, and no run in place linux versions are published)

Solutions

• Add a Flatpak manifest to the repository
• Build a Flatpak bundle in CI
• Upload the bundle as a build artifact

Alternatives

None

Additional context

Flatpak team have a good support of GitHub Actions:

• https://github.com/flatpak/flatpak-github-actions

I started experimenting with it to build Minetest and it works great.

Something that could be done later is publishing to Flathub directly from the GitHub CI, like OBS Studio does. The major limitation is that the use of QEMU is required for the ARM version until GitHub give access to ARM runners to everyone (currently it's a private beta)