Setting flow_run_id in local runs breaks slack notifier

[joshmeek]

#2868 made some additions to local runs so that targets/results could be templated properly both locally and in runs using a backend API. Setting flow_run_id on local runs here:

prefect/src/prefect/core/flow.py

Lines 964 to 969 in 30c88b3

	flow_run_context.update(
	scheduled_start_time=next_run_time,
	flow_id=self.name,
	flow_run_id=str(uuid.uuid4()),
	flow_run_name=str(uuid.uuid4()),
	)

breaks the slack_notifier because it has an extra check for a flow_run_id in context.

prefect/src/prefect/utilities/notifications/notifications.py

Line 151 in 30c88b3

if prefect.context.get("flow_run_id"):

I think this should be resolved by making the backend functionality of the slack notifier opt-in instead of default when a flow_run_id is present.