tests: use postgres container w/ pg_cron; lint checking now happens in its own job

[tazlin]

• As found in the new haidra-docker-images repo, the postgres service container is now a custom haidra-org authored one.
  • Currently, the only difference from the official postgres image is the installation and configuration of pg_cron. However, by having this new repo, we can easily modify the database image in the future and it makes it available to developers for local purposes or even possible third-party horde deployers.
• The lint/format check now happens in a parallel job so the substance of the code can be checked without the lint causing a premature failure of the entire workflow.
• An incidental style fix, which was missed due to the tests recently not working as intended.