systemd: Disable pipewire user services for root

The `access(2)` based multi-user mediation mechanism doesn't quite work
for the root user, which may cause it to conflict with a running
foreground user session. Prevent this by not running the user service at
all for the root user, which nobody should be doing anyway.

(cherry picked from commit 9bc29b4b37)

src/daemon/systemd/user/pipewire.service.in

@@ -14,6 +14,7 @@ Description=PipeWire Multimedia Service
 # After=pipewire.socket is not needed, as it is already implicit in the
 # socket-service relationship, see systemd.socket(5).
 Requires=pipewire.socket
+ConditionUser=!root
 
 [Service]
 LockPersonality=yes

src/daemon/systemd/user/pipewire.socket

@@ -1,5 +1,6 @@
 [Unit]
 Description=PipeWire Multimedia System Sockets
+ConditionUser=!root
 
 [Socket]
 Priority=6