gitlab-sshd: Handle SSL_CERT_DIR configuration correctly

The following discussion from !451 (merged) should be addressed:

• @theoretick started a discussion: (+1 comment)

  Not sure if this was in scope or how best to fix this one unfortunately 🤔

We call os.Setenv("SSL_CERT_DIR") once per SSH connection in gitlab-sshd. We use the same value each time (sourced from the configuration file), but it's possible that the first SSH connection operates without the benefit of this configuration.

We also need to decide whether an environment variable already set for SSL_CERT_DIR should (to be consistent) take precedence over the config-file value.