2017.4.29f1 fails to build with Gtk-WARNING **: 13:37:48.987: cannot open display:

As shown in !47 (merged), 2017.4.29f1 fails to build with following error:

(UnitySetup:12): Gtk-WARNING **: 13:37:48.987: cannot open display: 
The command '/bin/sh -c wget -nv ${DOWNLOAD_URL} -O UnitySetup &&     if [ -n "${SHA1}" -a "${SHA1}" != "" ]; then       echo "${SHA1}  UnitySetup" | sha1sum --check -;     else       echo "no sha1 given, skipping checksum";     fi &&     chmod +x UnitySetup &&     echo y |     ./UnitySetup     --unattended     --install-location=/opt/Unity     --verbose     --download-location=/tmp/unity     --components=$COMPONENTS &&     rm UnitySetup &&     rm -rf /tmp/unity &&     rm -rf /root/.local/share/Trash/*' returned a non-zero code: 1
ERROR: Job failed: exit code 1