Crank up borgbackup log level to INFO (from default WARN)

The default log level ("WARN") means borgbackup does not output
anything (and exits with a "0" return code) on success.

We'd like to have some more insight into long-running monthly
backup repo checks and turn up the log level to "INFO" with
this change.

See https://borgbackup.readthedocs.io/en/stable/usage/general.html#common-options .

Fixes #136.