MySQL8 fails for Munin on Ubuntu 20.04

MySQL 8 doesn't work in Munin on Ubuntu 20.04:

https://github.com/cyclestreets/cyclestreets-setup/blob/master/live-deployment/deploy.sh#L53-L80

`
mysql_                     | no   | no [DBI connect('mysql;mysql_read_default_file=/etc/mysql/debian.cnf;mysql_connect_timeout=5','root',...) failed: Access denied for user 'root'@'localhost' (using password: YES)]
`