fix: core startup fails if /opt/mailman/mailman-extra.cfg is read-only

[pgmillon]

Follow-up to #86, after studying the content of /opt/mailman, only the var sub-folder seems important as writable.

[maxking]

Thanks for the fix, LGTM! Yes, Mailman really only needs to write inside the /var directory.

[pgmillon]

I don't know why but for some reason CircleCI doesn't trigger 🤔

[maxking]

I am investigating that, somehow this PR didn't trigger circleci but few before this did.

[maxking]

Ok, now the tests ran when it got merged with main and is failing. The reason is mostly that var may not exist during first startup since it is created only after mailman starts.

We might be able to pre-create a var directory and have Mailman use it (idk if it will complain about sub-directories missing or will just create it - we can try it out though).