use CRON environment variable to suppress non-error messages from gitlab backup task

The /etc/backup.d/10_backup-gitlab.sh script invoked as a backupninja action, calls the /usr/bin/gitlab-backup bash script which in turn is a wrapper fro invoking the rails rake command /opt/gitlab/bin/gitlab-rake gitlab:backup:create . As per the documentation documentation setting the CRON environment variable directs the backup script to hide all progress output if there aren’t any errors.