[debian package] failed to invoke rollback watcher

[leehinman]

If elastic-agent is installed via a DEB package, it cannot find the executable when it tries to start the watcher.

failed to invoke rollback watcher: failed to start Upgrade Watcher: fork/exec /var/lib/elastic-agent/elastic-agent: no such file or directory

This is because the path library doesn't know that it installed via the package manager and doesn't point to the correct symlink which is /usr/share/elastic-agent/bin/elastic-agent

We need to either not try to start the watcher under a debian package or get the path correct.

See #4301

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)