Fix harmless exception in port DB script (#15814)

The port DB script would try and run database background tasks, which
could fail if the data they acted on was in the process of being ported.
These exceptions were non fatal.

Fixes #15789

changelog.d/15814.misc

@@ -0,0 +1 @@
+Fix harmless exceptions being printed when running the port DB script.

synapse/_scripts/synapse_port_db.py

@@ -1369,6 +1369,9 @@ def main() -> None:
         sys.stderr.write("Database must use the 'psycopg2' connector.\n")
         sys.exit(3)
 
+    # Don't run the background tasks that get started by the data stores.
+    hs_config["run_background_tasks_on"] = "some_other_process"
+
     config = HomeServerConfig()
     config.parse_config_dict(hs_config, "", "")