Don't create config template if alternate path used
From the documentation, config files in many locations are supported:
These file system paths are searched in the following order to find your site's custom
mailman.cfg
file. The first file found is used.
- The file system path specified by the environment variable
$MAILMAN_CONFIG_FILE
mailman.cfg
in the current working directoryvar/etc/mailman.cfg
relative to the current working directory$HOME/.mailman.cfg
/etc/mailman.cfg
/etc/mailman3/mailman.cfg
../../etc/mailman.cfg
relative to the working directory ofargv[0]
However, the check before writing the config template only checked if /etc/mailman.cfg existed before writing the template, so the template would always be written for users or alternative config file paths, even when they had existing configurations.