This adds a new runner called `org.osbuild.blank`, which assumes /usr
is pre-populated and ready to go. It does not perform any OS setup. It
only initializes the environment and executes the stage.
This runner allows easy bootstrapping of new systems. It assumes our
ideal setup where `/usr` describes a host system in its entirety,
without any local policy applied. Thus, this runner is also what we
ultimately want to work towards as a default. This might not happen
anytime soon, though, given how `passwd`, `ldconfig`, `nss`, etc. still
depend on prepopulated caches in `/etc`.
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