Support path-triggering when using composefs backend

We have a collection of systemd units that trigger on modification of `/ostree/bootc` today when using the ostree backend. Most notably this triggers rhsm facts generation whenever deployments are changed, either while modifying a running system or when the deployments change during boot (booted becomes rollback, staged becomes booted).

We don't handle that at all with the composefs backend currently, so we'll need to (1) add the code in the right place to update the mtime on the composefs storage root, and (2) update the systemd unit(s), I am pretty sure this is simply adding an extra `PathChanged` line to the .path unit and everything else should continue functioning the same.