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.

[1] https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/commit/24928d6b6fa1cde3312bcf63e4e14d7003aadc69