In order to use it, the environment variable must be explicitly set.
Otherwise, the plugin is pulled and activated by any Qt application.
The behavior now matches the documentation.

Task-number: QTRD-3660
Task-number: QTBUG-47099
Change-Id: Iaa09c7646bfe6c4de898ba03940af3de2fb3d4a7
Reviewed-by: Jarkko Koivikko <jarkko.koivikko@code-q.fi>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>