"Environment Override" is inaccurate on the Status report page when "Require environment override" is enabled and an environment_override variable is set

Problem/Motivation

When the environment_require_override option is enabled, the Status report page says "You should override the 'environment_override' variable in your settings.php file to indicate the server environment this site is in." regardless of if that variable is set or not.

Steps to reproduce

1. Install Drupal with the Environment module.
2. Navigate to the settings form at admin/config/system/environment/
3. Enable "Require environment override" and click "Save".
4. Navigate to the Status report page at admin/reports/status/. It should currently contain the message quoted above under "Environment Override".
5. Add $conf['environment_override'] = 'development'; to settings.php.
6. Clear cache and refresh the Status report page. It should no longer contain the message from #4, but it does.

This is because environment_requirements() (environment.install) always gives that message if the option from #3 is enabled.

Proposed resolution

Only give that message within environment_requirements() when the environment_override variable is not set in settings.php. When it is, return an OK status message.

Remaining tasks

I'll be attaching a patch that achieves the proposed resolution.