* ci: Switch Flatpak build to flathub-infra/flatpak-github-actions
Flathub team has recently forked flatpak-github-actions and merged
various PRs submitted to the original repo. However, it's not versioned
(yet?), so pin the latest commit instead.
Additionally, enable validation of the build using flatpak-builder-lint,
and run all steps in the container with the runtime and required tooling
baked in.
* Update mirror-screenshots-url
* Shush git complaining about "dubious" ownership
* Update date format to iso8601
* Fix flatpak-builder-lint invocations