Update environment-variables.md

[falegh]

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[fivethreeo]

Needs a new section for config env vars.

[anthonyp-usafacts]

Needs a new section for config env vars.

What exactly did you mean here?

[fivethreeo]

The first section shows variables you can use in code that are set by razzle.

Should have another section that shows variables you can set to affect razzle behaviour and config.