The default env file create by the Vercel CLI should match the Next.js recommended filename

[ijjk]

Currently the default filename with vercel env pull is .env and Next.js recommends .env.local and makes sure this .env filename variant is already ignored in .gitignore in the templates. We should update the default for the CLI to match this so the additional filename isn't needed (docs ref)

IMPORTANT: This needs to be tackled in a non-breaking way.