generator: Exit if there's no `/run/ostree`

generator: Exit if there's no `/run/ostree`

Currently if run in a container image under systemd, we will
incorrectly synthesize a `var.mount` unit even if `ostree-prepare-root`
hasn't run.

The comment here said why we didn't do that before, but that's
for the really legacy embedded-only "ostree-prepare-root-static"
path, and even then I'm pretty sure it was wrong because
the generator here only runs in the *real* root, and we should
have `/run/ostree` at that point.