community/pipewire: fix issue with autostarting on some DEs

Some desktop environments that use gnome-session cannot start pipewire in the 'Initialization' phase, and the phase times out waiting for pipewire. Moving it to a later phase allows it to start properly, 'Panel' is the earliest phase where this works as expected at least on the DE (Phosh) where the original issue was encountered.

Also see: https://gitlab.com/postmarketOS/pmaports/-/issues/963