resilient/runner_exporter: use --keep-dirlinks instead of --safe-links.

so that we can move part of backup directories to another partition like :

• ~/srv/backup/theia/srv/runner/instance/slappart4 -> /media/mariadb/theia-backup/slappart4
• ~/srv/backup/theia/srv/runner/instance/slappart5 -> /media/zodb/theia-backup/slappart5