Change NODE_ENV to APP_ENV

[codemist]

References:

Jira: MNTOR-
Figma:

Description

Screenshot (if applicable)

Not applicable.

How to test

Checklist (Definition of Done)

• Localization strings (if needed) have been added.
• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• I've added a unit test to test for potential regressions of this bug.
• If this PR implements a feature flag or experimentation, I've checked that it still works with the flag both on, and with the flag off.
• If this PR implements a feature flag or experimentation, the Ship Behind Feature Flag status in Jira has been set
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[github-actions]

Preview URL 🚀 : https://blurts-server-pr-5666-mgjlpikfea-uk.a.run.app

[github-actions]

Cleanup completed - database 'blurts-server-pr-5666' destroyed, cloud run service 'blurts-server-pr-5666' destroyed