make a wrapper so that dunst can find dbus-launch

Prior to this change, the following would fail
  nix-shell --pure -p dunst --command dunst
while this would succeed
  nix-shell --pure -p dunst -p dbus_daemon --command dunst

because dunst tries to execve 'dbus-launch'. The same issue hit if dunst
was used inside a (user) systemd service (which is what I actually care
about).