Work around inkscape accessing its environment.

Work around inkscape accessing its environment.

Bug #997904 points out that even this command-line use of Inkscape
will try to write to ~/.config/inkscape (in order to create a default
one if it's not already there), and also, will try to initialise GDK
(especially strange in the absence of an X or Wayland server, where it
will try to fall back to the HTTP-server based Broadway backend).

Worked around by resetting $HOME to the build directory, where it
doesn't matter if we create an Inkscape config directory, and by
setting GDK_BACKEND to the empty string, which seems to inhibit it
trying to actually launch any backends.