Disable PostgreSQL warning logging in CI

[mikkonie]

Due to an issue within the iRODS server, certain actions raise there is no transaction in progress warnings in postgres. These are also visible at the end of each CI run. Not harmful but annoying especially when going through the logs for debugging something.

In docker compose this can removed with command: ["postgres", "-c", "log_min_messages=error"]. Need to figure out a similar syntax in GitHub Actions.

See also bihealth/sodar-docker-compose#17.