workflows: clone flathub repository from flathub
workflows: clone flathub repository from flathub

Instead of cloning our fork of the flathub repository, clone the
"official" one under the flathub project.  We still push the new branch
to our own repository, however.

This prevents us from having to keep our own repository up to date.  In
fact, our main branch now contains only a README telling you to look
elsewhere.

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