flatpak: add a publish_beta template job
This is not meant to be used as part of every CI pipeline, but instead on special occasions like when tagging a beta release.
Ideally there are special requirements for pushing to flathub-beta that you need to make sure you follow if you want to use this. Whats needed is the following:
- Ask a sysadmin to add a FLATHUB_BETA_TOKEN to your project and make sure your branch is protected
- Make sure you application depends on a runtime found in Flathub or Flathub beta. You shouldn't use this with the Nightly runtime.
- Make sure your 'branch' in the Flatpak manifest is beta
- Change only: and when: yaml keys to fit your needs
Close #6