Replace dotenv with @next/env in neon guide

[kevinzunigacuellar]

[!IMPORTANT] 🔎 Previews:

-

Explanation

This PR removes the usage of dotenv from the neon guide. While a small change, it favors the use of framework-specific utility libraries and reduces unnecessary dependencies, thus simplifying the overall setup.

Changes Made

• Replaced dotenv with @next/env utility for loading environment variables.
• Added a check for DATABASE_URL in drizzle.config.ts.

[kevinzunigacuellar]

Let's remove dotenv and next/env altogether, keep in the second check for the DB URL.

Thanks for the review Roy

Unfortunately, removing next/env from the drizzle.config file will cause an error because the Drizzle ORM CLI doesn’t rely on the Next.js server to run scripts, so these variables won't be available at runtime.

While it is possible to remove next/env from schema.ts, it’s a common practice to load environment variables at this point for scripts that run outside the Next.js server context.

[kevinzunigacuellar]

Thanks for the support @royanger 🎉