test_export_data_command.sh CI script hung #11189

I had an instance of the test_export_data_command script running for 6h in CI before eventually being timed out: https://github.com/matrix-org/synapse/runs/4010916318?check_suite_focus=true

It looks like the script got to this line before simply stopping:

Perhaps a case of Synapse failing to shut down properly?

Should we place a smaller timeout on (a part of) this script - which would fail the test, but without burning so much CI time - in the short term?