Don't use working directory when --output-directory option is used

Otherwise when running something like:

  /root/system-info/grml-hwinfo --output-directory=/root/system-info/grml-hwinfo-data

via cron this store data to /root/root/system-info/grml-hwinfo-data
(instead of the requested /root/system-info/grml-hwinfo-data) since
/root is the working directory for cron.