progress: do not set run_finished_at for stopped workflows

[mdonadoni]

After 199c163, the run_finished_at field of stopped workflows is now being set, even though it should be None.

This is due to the fact that the status update for stopped workflows is now being handled by _update_workflow_status, which manually sets run_finished_at:

reana-workflow-controller/reana_workflow_controller/consumer.py

Line 151 in 1dc2116

workflow.run_finished_at = datetime.now()