Bug Report: cron.database_name is not dynamic; fixed to circle_test

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**Describe the bug**
POSTGRES_DB and the sample `circle_test` db in custom-postgresql.conf should be set to a variable, not hard coded as circle_test.

this would also apply to other options like listen addresses and shared_preload_libraries

As a result, even setting POSTGRES_DB to `something`, would not enable an extension like pg_cron, because it would only be enabled for circle_test

**Expected behavior**
changing POSTGRES_DB should also affect cron.database_name

**Workarounds**
editing the custom-postgresql.conf manually

**Screenshots and Build Links**
If possible, add screenshots and links to jobs to help explain your problem.

**Additional context**
Add any other context about the problem here.