rubocops/lines: check allowlist for use of ENV.runtime_cpu_detection

[carlocab]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
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• Have you successfully run brew tests with your changes locally? (In progress... will update this when it's done.)

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This is implements the first audit discussed in #11608. (Second audit is in progress.)

[BrewTestBot]

Review period will end on 2021-07-02 at 17:53:37 UTC.

[MikeMcQuaid]

Perfect, thanks @carlocab!

[BrewTestBot]

Review period skipped due to critical label.