Fixes #810.

Description

This PR adds:

1. A warning message about environment variables used by conda-store to override user conda configuration, which should help clear up user uncertainty about the output of conda config.
2. Some information in the documentation about the above ☝️
3. An info-level log message is now emitted displaying the amount of time each action takes.

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?