The php installer creates a random one, but we bypass it, so we have
to create one ourselves.
This should be backward compatible as encryption is used for session
cookies only: users at the time of the upgrade will be logged out but
nothing more.
259b7fa065/config/config.inc.php.sample (L73)
If the database is local, use postgres peer authentication.
Otherwise, use a password file.
Leave database initialisation to postgresql.ensure*.
Leave /var/lib/roundcube creation to systemd.
Run php upgrade script as unpriviledged user.
When using a different database, the evaluation fails as
`config.services.postgresql.package` is only set if `services.postgresql` is enabled.
Also, the systemd service shouldn't have a relation to postgres if a
remote database is used.
With this option it's possible to specify a custom expression for
`roundcube`, i.e. a roundcube environment with third-party plugins as
shown in the testcase.
