borg-client: Create timestamp file on backup host after successful backup

This allows to easily check for backups with no update to the timestamp
on the backup host.

Signed-off-by: Florian Pritz <bluewind@xinu.at>

roles/borg-client/templates/borg-backup.sh.j2

@@ -9,3 +9,5 @@ borg create -v --stats -C lz4 -e /proc \
     -e /sys -e /dev -e /run -e /tmp -e /var/cache \
     {{ backup_host }}:{{ backup_dir }}::$(date -I) /
 borg prune -v {{ backup_host }}:{{ backup_dir }} --keep-daily=7 --keep-weekly=4 --keep-monthly=6
+
+ssh {{backup_host}} touch '{{backup_dir}}/last-backup-timestamp'