Flatpak support

[jimafisk]

I'm opening this as a placeholder because I imagine some folks might want to use flatpak instead of snap. If that's the case, we'd need Goreleaser to add support before we can add it to this project. If that's something you'd like to see, I think the best course of action would be to voice your opinion on this issue to show the maintainer that there's interest in this: goreleaser/goreleaser#2927