Add support for snapped pipewire

A snapped pipewire requires being able to create the pulse
folder inside the global XDG_RUNTIME_DIR folder (not the snap
own folder). Also, it requires to be able to create both the
pipewire-[0-9]-manager and its lock file.

This patch adds that capability to the audio-playback slot;
thus, a snap offering that slot will be able to create the
corresponding global sockets.

interfaces/builtin/audio_playback.go

@@ -116,9 +116,11 @@ owner /{,var/}run/pulse/** rwk,
 
 owner /run/user/[0-9]*/ r,
 owner /run/user/[0-9]*/pulse/ rw,
+owner /run/user/[0-9]*/pulse/** rw,
 
 # This allows to share screen in Core Desktop
 owner /run/user/[0-9]*/pipewire-[0-9] rwk,
+owner /run/user/[0-9]*/pipewire-[0-9]-manager rwk,
 
 # This allows wireplumber to read the pulseaudio
 # configuration if pipewire runs inside a container