recipes-connectivity/avahi/avahi-daemon/avahi-daemon-override.conf

[Service]
ExecStartPre=/bin/sh -c "grep -q avahi: /etc/group || /usr/sbin/addgroup --system avahi"
ExecStartPre=/bin/sh -c "grep -q avahi: /etc/passwd || /usr/sbin/adduser --system --no-create-home --ingroup avahi avahi"