Add support for starting envbuilder from a built image

[mafredri]

To fully support starting envbuilder built images from cache, we need to also support starting envbuilder to setup the environment and run the init command without making changes to the filesystem.

We already have a facility for this when envbuilder detects the presence of /.envbuilder/built and --skip-rebuild=true is used. However, providing --skip-rebuild=true should not be necessary.

We may introduce a new indicator file (e.g. constants.MagicImage = "/.envbuilder/image") to detect envbuilder built images.