Repair wp/7.0 branch Actions: remove SimenB/github-actions-cpu-cores dependency

[desrosj]

Part of #82782.

What?

Removes the SimenB/github-actions-cpu-cores third-party action dependency from unit-test.yml, matching upstream trunk's own removal of it.

Why?

Upstream trunk removed this dependency in 4d02abd34c61e8bcf30399c219fdccedaf0e8a84 (see #78593). The action was only ever used to populate a MAXWORKERS environment variable fed into Jest's --maxWorkers flag. nproc, run directly on the runner, provides the exact same value without depending on an external action.

This branch (wp/7.0) still had the dependency in three jobs in unit-test.yml: unit-js, unit-js-date, and the currently-disabled mobile-unit-js. Each had its SimenB/github-actions-cpu-cores step (and id: cpu-cores) removed, its MAXWORKERS env var line dropped, and its --maxWorkers="$MAXWORKERS" usage switched to --maxWorkers="$(nproc)" directly.

Use of AI Tools

This PR was created by Claude Code under my supervision. All code should be treated as AI-produced and not yet reviewed by a human until this PR is marked Ready for Review.

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🟢 Approval recommended

All affected jobs use Ubuntu runners where nproc is available, preserving existing worker-count behavior.

Pull request overview

Removes an unnecessary third-party CPU-counting action from JavaScript unit-test jobs.

Changes:

• Removes SimenB/github-actions-cpu-cores steps and MAXWORKERS.
• Uses nproc directly for Jest worker counts.

File summaries

File	Description
.github/workflows/unit-test.yml	Replaces action-provided CPU counts with nproc.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

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

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