[DEV-15465] stop passing download_database_url in command line

[james-at-kc]

Description:

Currently we pass the settings.DOWNLOAD_DATABASE_URL Postgres connection string to the download workers when starting a download. This connection string contains the username and password which would be readable to anyone on the server by looking at the running processes. An alternative could be to use the AWS Secrets Manager to retrieve the connection string on the workers.

We also do the same thing with the monthly-delta-job in Jenkins.

Technical Details:

Used temp env variables to hide the credentials. No AWS secrets are involved.

Requirements for PR Merge:

1. Unit & integration tests updated
2. N/A - API documentation updated (examples listed below)
   1. API Contracts
   2. API UI
   3. Comments
3. N/A - Data validation completed (examples listed below)
   1. Does this work well with the current frontend? Or is the frontend aware of a needed change?
   2. Is performance impacted in the changes (e.g., API, pipeline, downloads, etc.)?
   3. Is the expected data returned with the expected format?
4. N/A - Appropriate Operations ticket(s) created
5. Jira Ticket(s)
   1. DEV-15465

Explain N/A in above checklist:

2 - No API change has occurred, so no API documentation update needed.
3 - No Data change was made, so no impact on frontend nor performance
4 - No operations involvement required other than the deployment of this branch.