Fix Audit check for env :std

[dunkmann00]

When performing a strict audit of a formula in the core tap, it was
possible that the check for env :std would not happen.

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This fixes the strict audit, making sure that if the formula is part of core, brew audit --strict checks to see if env :std is used.

This references #6737.

[scpeters]

Nice catch!

Given that the return unless disguised this issue, do you mind changing return unless line.include?("env :std") as well to use an if statement?

[dunkmann00]

Sure thing! Should I squash the 2 commits together?

[MikeMcQuaid]

Yes please!

[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @dunkmann00!

[dunkmann00]

Thanks!