because we're setting DISPLAY always, and under wayland that resolves to
"wayland", not the original ":0" so the gtk2 gvfs-open eventually called open
xdg-open cannot open the correct display

(cherry picked from commit 3bb7557d)

Change-Id: I246120f2430b92cd2d3e5003445aa4c9da4f6a68
Reviewed-on: https://gerrit.libreoffice.org/20721Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>