Also use the GERRITHOST variable instead of magic text.

Change-Id: I3314be81017f64fca8aa756cca1715081ad01641
Reviewed-on: https://gerrit.libreoffice.org/41988Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>