launch: avoid autostarting pipewire-pulse systemd units for root
Long story short, this is something pulseaudio has since 2017 [1]. If pipewire-pulse is aiming to be a drop-in replacement, the same behaviour is probably expected.
I noticed that without this line I hit some issues with systemd socket activation. In fact, it does not work properly at all. It probably has something to do with the fact that pipewire-pulse is started for GDM session and is somehow propagated down to my own user session.