x11-servers/xwayland-devel: allow concurrent instance after 1c4b6bc295c6

wlroots >= 0.16 spawns Xwayland with -displayfd. Starting a new
compositor re-created /tmp/.X11-unix/X0 thus breaking X11 connection
to an already running compositor.

Regressed by: https://gitlab.freedesktop.org/wlroots/wlroots/-/commit/4741e9d8410f

(cherry picked from commit fb41bd9ef0438118ccc7e2e43d952fc8c2adcd63)