Rollback cleaner job runs for unupgradable agents

#11562 added a periodic rollback cleaner job, and #14378 made it check the versioned home symlink. However, it doesn't disable itself if the agent isn't upgradable - for example it's running in a container - and instead periodically emits a log line about not being able to resolve the symlink:

```
reading symlink "/usr/share/elastic-agent/state/data/elastic-agent": readlink /usr/share/elastic-agent/state/data/elastic-agent: no such file or directory
could not resolve live versioned home symlink during cleanup; orphan directories will be kept conservatively"

"cleanup completed in degraded state; 0 unexpired rollback(s) preserved; rescheduling in 10m0s"
```

The rollback cleaner should not run at all if agent isn't upgradable, much like the upgrade watcher.