Add CRON_TZ or similar timezone support

### Summary

Does the crontab for Graphile Worker support time zone?

### Additional context

Our customers are from New York mostly. We'd like to create a cron job that runs daily at 8am New York time to send notifications to customers. We'd like to be agnostic to whether it's EDT (GMT-4) or EST (GMT-5). Is there a way to do that? I don't want to have to change the crontab file twice a year whenever day light savings kicks in.

I believe the standard `crontab` supports `CRON_TZ` entry but not sure if it's supported by Graphile Worker.

Thanks!