v2022dev1 ← v2022dev0 forward port: Automatic KVM/UML backend selection for Debos/Fakemachine

Forward port of !152 (merged)

• 841b271c Revert "Drop the APT GPG key from images that get it via the base container"
• 802c2285 {image,flatpak}-builder: Install packaged version of debos
• 2372ac54 image-builder: test-recipe: Check fakemachine backend
• cc46d887 image-builder: Run smoke test on shared runner
• fe76b97d image-builder: Run smoke test on kvm runner