fix: Don't overwrite .env.local if one exists

[davidianstyle]

Description

Unbreaks incorrect overwriting of environment by skipping copy of .env.local if .env or .env.local already exist.

Checklist

• I have installed pre-commit and installed the hooks with pre-commit install before creating any commits.
• I have updated any related tests for code I modified or added new tests where appropriate.
• I have manually tested my changes as follows:
  • cd frontend
  • cp .env.dist .env
  • Set NEXT_PUBLIC_BACKEND_URL to anything other than localhost
  • cd ../docker
  • docker compose -f docker-compose.prod.yml -f docker-compose.yml up
  • Make sure the front end requests from the URL you specified in NEXT_PUBLIC_BACKEND_URL as opposed to localhost
• I have updated any relevant documentation or created new documentation where appropriate.