feat(env): add .local variants to defaultEnvFiles (#1604)

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Summary

Fixes #1604

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[danez]

Thank you for the PR and for improving the Netlify CLI. 🦄

Even though you are able to specify custom env files in the netlify.toml file I think it makes sense to adopt the defaults.

For reference here are the docs of nextjs around this: https://nextjs.org/docs/basic-features/environment-variables

[github-actions]

📊 Benchmark results

Comparing with f83a930

Package size: 230 MB

⬇️ 0.00% decrease vs. f83a930

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