Load flows for deployments in a worker thread (#6340)

If not run in a worker thread, it is executed in an "async" context which means that any methods decorated by `sync_compatible` will return a coroutine. However, top-level code in a script is always intended to be run in a "sync" context, so the user can never `await` the coroutines and code that works without a deployment will break while using a deployment.

src/prefect/deployments.py

@@ -18,6 +18,7 @@
 from prefect.infrastructure import DockerContainer, KubernetesJob, Process
 from prefect.logging.loggers import flow_run_logger
 from prefect.orion import schemas
+from prefect.utilities.asyncutils import run_sync_in_worker_thread
 from prefect.utilities.callables import ParameterSchema
 from prefect.utilities.dispatch import lookup_type
 from prefect.utilities.filesystem import tmpchdir
@@ -54,7 +55,7 @@ async def load_flow_from_flow_run(
     if deployment.manifest_path:
         with open(deployment.manifest_path, "r") as f:
             import_path = json.load(f)["import_path"]
-    flow = import_object(import_path)
+    flow = await run_sync_in_worker_thread(import_object, import_path)
     return flow