workflows: new update-flathub.yml
workflows: new update-flathub.yml

This is a somewhat hacky first attempt to semi-automate the flathub
release process.

This workflow downloads the tarball from GitHub and bases the checksum
on that.  Ideally we'll have this as part of the release process at some
point, and calculate the checksum based on that.

cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit