scripts: ci: check_compliance: do not use git mailmap

When looking at git author in a commit range using git log, explicitly
indicate we don't want to use the mailmap file (default git client
behavior is to use it), so that the "actual" author info from the commit
is used to perform the compliance check, and not whatever identity the
author may have mapped to in the mailmap file (ex. their "main" email
address might be different from the one they used to commit).

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>