Add Prisma env file path to secret.config

[sherryhli]

Ticket link

N/A

Implementation description

• Added Prisma env file path to secret.config

Steps to test

1. If you have an existing backend/prisma/.env file, delete it
2. Run the Vault secret update script and check that you have a backend/prisma/.env file and that it is populated with the expected DATABASE_URL

vault kv get -format=json kv/sistering | python update_secret_files.py

What should reviewers focus on?

• N/A

Checklist

• My PR name is descriptive and in imperative tense
• My commit messages are descriptive and in imperative tense. My commits are atomic and trivial commits are squashed or fixup'd into non-trivial commits
• I have run the appropriate linter(s)
• I have requested a review from the PL, as well as other devs who have background knowledge on this PR or who will be building on top of this PR