stderr gets merged into stdout

Unfortunatelly xvfb-run merges the stderr of librepcb-cli into stdout, this is very annoying when parsing the output of the CLI.

See https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1059947 or https://debian-bugs-dist.debian.narkive.com/lXGukaHu/bug-868876-xvfb-run-2-1-breaks-wkhtmltopdf