Remove dependency used for counting available CPUs

[desrosj]

What?

This removes the SimenB/github-actions-cpu-cores action as a dependency.

Introduced in 5571a79 by way of #59904.

Why?

The action sets the result of a single expression as an output for future steps to make use of.

With supply chain attacks occurring more and more frequently, it's in the project's best interest to limit the number of dependencies a repository has as much as possible. This is a good example of "a dependency for dependency's sake" as it can be replaced with a single simple command.

How?

Since every job using this action is Linux-based, the same can be accomplished using nproc.

Use of AI Tools

I used Claude to analyze how the dependency is used within the repository and evaluate whether it can safely be removed in favor of a simpler solution.

[github-actions]

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Co-authored-by: desrosj <desrosj@git.wordpress.org>
Co-authored-by: johnbillion <johnbillion@git.wordpress.org>

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[Copilot]

Pull request overview

This PR removes the third-party SimenB/github-actions-cpu-cores GitHub Action from the Unit Tests workflow and replaces it with the built-in nproc command to determine CPU core count for Jest’s --maxWorkers on Linux runners, reducing supply-chain dependencies.

Changes:

• Removed the SimenB/github-actions-cpu-cores action steps from the unit test workflow jobs.
• Replaced use of the action output (steps.cpu-cores.outputs.count) with $(nproc) when passing --maxWorkers to Jest runs.

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[github-actions]

Flaky tests detected in 826d9f0.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/26307760071
📝 Reported issues:

• [Flaky Test] should move focus back to the Publish panel toggle button when canceling #77721 in /test/e2e/specs/editor/various/publish-panel.spec.js
• [Flaky Test] shouldn't conflict with server-side autosave #35176 in /test/e2e/specs/editor/various/autosave.spec.js

[johnbillion]

Runs with maxWorkers set to 4, same as it is in trunk.

npm run --workspace @wordpress/unit-tests test:unit -- --ci --maxWorkers=4 --shard=1/4 --cacheDirectory=/home/runner/.jest-cache