Explicitly checking that EXTEND_ESLINT is true

[BrockWills]

The documentation for the new EXTEND_ESLINT env variable says it applies When set to true but the code only checks existence of the variable.

Setting EXTEND_ESLINT=false still enables extending eslint since process.env.EXTEND_ESLINT is technically set.

This PR just checks that process.env.EXTEND_ESLINT === 'true'

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[ianschmitz]

Good catch. Thanks!

[PaulRBerg]

It's weird, but in my case I don't have any EXTEND_ESLINT env var set and I can override the config easily via a custom .eslintrc. I always thought that this is the default behaviour until a different problem lead me to this thread.

Using cra 3.1.0 and yarn 1.17.3.

[silverwind]

@PaulRBerg Third party tools that directly read .eslintrc always worked, but the linting step executed via react-scripts will ignore the file unless they run with EXTEND_ESLINT=true.