The execute function should not generate allure items in RobotFramework provider if allure report generation is disabled

Current situation

Allure reports are now disabled by default, but the Robot Framework still generate the allure artefacts in this case. This unnecessarily consumes resources.

Desired outcome

The robotframework/execute function should not attempt to produce allure artefacts if OPENTF_ALLURE_ENABLED is not set (yes, true, on).