🔒 Fix minor data exposure vulnerability

Full report:
https://github.com/rugk/borg-cron-helper/wiki/Minor-vulnerability:-Data-exposure-with-borg-cron-helper---1.0.1

Thanks to @kinafu for the report.

borgcron.sh

@@ -103,7 +103,7 @@ if [ "$BACKUP_NAME" = "" ] ||
 	echo 'Some required variables may not be set in the config file. Cancel backup.'
 	exit 1
 fi
-if ! export|grep "BORG_REPO"; then
+if ! export|grep -q "BORG_REPO"; then
 	echo 'The BORG_REPO variable is not exported in the config file. Cancel backup.'
 	exit 1
 fi