v5: Determine build CPU count dynamically via cgroups

[mathomp4]

This PR replaces the static nproc call with a dynamic cgroups-based (v1 and v2) calculation to accurately determine the actual allocated CPU limits on CircleCI runners.

[mathomp4]

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[mathomp4]

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[mathomp4]

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[mathomp4]

Your development orb(s) have been published. It will expire in 30 days.
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[mathomp4]

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