text: Title: Do not override already set env variables from `.env`
  Body: ## Problem
  
  <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
  
  Currently the `.env` variables take precedence over the system environment variables, this can cause issues as the Prisma CLI will use the system environment variables instead which could lead to migrations being applied to a different database if you have two different connection strings set.
  
  ## Suggested solution
  
  <!-- A clear and concise description of what you want to happen. -->
  
  System environment variables should take priority.
  
  ## Additional context
  
  Mentioned in #420.