v2021 ← v2022dev0 backport: Automatic KVM/UML backend selection for Debos/Fakemachine

Backport of !152 (merged)

• d355ea56 Revert "Drop the APT GPG key from images that get it via the base container"
• 2ebeafef {image,flatpak}-builder: Install packaged version of debos
• 7d4a500c image-builder: test-recipe: Check fakemachine backend
• 5255be3e image-builder: Run smoke test on shared runner
• 584c06e2 image-builder: Run smoke test on kvm runner

changed the description

changed target branch from apertis/v2022dev1 to apertis/v2021

enabled an automatic merge when the pipeline for 584c06e2 succeeds

merged