Comment 0 for bug 1959375

Currently doing something like:

    sudo usermod -a -G snap_microk8s dbeamonte

on a Ubuntu Core system will fail with

    usermod: /etc/group.15965: Read-only file system

This is because the existing usermod patches to detect
the extrausers file do not cover this case. Attached
a simple patch that enables it. I will give this patch
a test run in our image PPA for jammy and if things look
good I would like upload to 22.04 and SRU for 20.04 and
18.04.