[!695] Make inkscape run completely headless in batch mode

Merge branch 'bug/make-inkscape-run-headless-on-macos' into 'master'

ref:core/dune-common We run inkscape in headless mode to convert a number of
SVG files. On macOS, inkscape unfortunately still wants to start the X server
and open a window in this mode. This patch adds option "-z" to override that
behavior.

See merge request [core/dune-common!695]

  [core/dune-common!695]: gitlab.dune-project.org/core/dune-common/merge_requests/695