Most Wayland compositor rely on the environment variable XDG_RUNTIME_DIR
to be set and the corresponding directory to be accessible.

If XDG_RUNTIME_DIR isn't set (as with some plain CI docker setups),
create a temporary directory and set the environment variable
accordingly.

Closes: https://gitlab.freedesktop.org/ofourdan/xwayland-run/-/issues/3
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>