This fixes a problem if only `SSL_CERT_DIR` were specified, we would get
an HTTPS failure since the default paths were not used
(https://github.com/excon/excon/pull/772).

This came out of
https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6616.

Diff: https://my.diffend.io/gems/excon/0.89.0/0.90.0

Changelog: fixed