Create React AppにおけるNODE_ENV
Create React AppにおけるNODE_ENVはコマンドごとに固定されている
https://create-react-app.dev/docs/adding-custom-environment-variables/
When you run npm start, it is always equal to 'development',
https://github.com/facebook/create-react-app/blob/6f3e32e3b16e1f8bc72cf45e0e39d8cd20685d02/packages/react-scripts/scripts/start.js#L12-L13
when you run npm test it is always equal to 'test', and
https://github.com/facebook/create-react-app/blob/6f3e32e3b16e1f8bc72cf45e0e39d8cd20685d02/packages/react-scripts/scripts/test.js#L12-L13
when you run npm run build to make a production bundle, it is always equal to 'production'.
https://github.com/facebook/create-react-app/blob/6f3e32e3b16e1f8bc72cf45e0e39d8cd20685d02/packages/react-scripts/scripts/build.js#L12-L13
利用者はoverrideすることはできない
誤ったビルド成果物をproductionにshipしないため