jibri: switch to PulseAudio

Using PulseAudio means no longer needing to mount /dev/snd/ which should
make deploying Jibri much easier.

Credits:

https://github.com/openfun/jibri-pulseaudio
https://community.jitsi.org/t/tip-pulseaudio-support-for-jibri/65780
https://github.com/kpeiruza/jitsi-images

jibri/rootfs/etc/services.d/30-pulse/run

@@ -0,0 +1,4 @@
+#!/usr/bin/with-contenv bash
+
+HOME=/home/jibri
+exec s6-setuidgid jibri /bin/bash -c "exec /usr/bin/pulseaudio"