ci: use cheaper ARM bootstrap runner for check-skip

[thepastaclaw]

Summary

Move only the check-skip bootstrap job onto a cheaper ARM runner path.

Problem

check-skip currently uses the generic amd64 runner setting, so even the
lightweight bootstrap step can consume the more expensive primary runner pool.

Solution

• add a dedicated RUNNER_CHECK_SKIP override for the check-skip job
• default that job to GitHub-hosted ARM (ubuntu-24.04-arm)
• leave downstream dynamic runner selection unchanged for the heavier CI jobs

This keeps the tiny bootstrap step cheap by default while still allowing an
org-level Blacksmith ARM label, such as a small 1 vCPU runner, to be
configured later without affecting the main build/test jobs.

Related

• split out from ci: migrate primary CI jobs to ARM runners #7232

Validation

• parsed .github/workflows/build.yml successfully with PyYAML after the
  change
• reviewed the workflow context around check-skip,
  select_dynamic_runner.py, and cache-depends-sources.yml
• ran the mandatory code-review gate on
  upstream/develop..ci/check-skip-arm-bootstrap → ship

[github-actions]

✅ No Merge Conflicts Detected

This PR currently has no conflicts with other open PRs.

[thepastaclaw]

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[thepastaclaw]

Review Gate

Commit: 229d466d

• ⏳ Debounce: 6m ago (need 30m)

• ⏳ CI checks: checks still running (10 pending)

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

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Run ID: ec233021-ad0e-4ff0-8835-1efce4d209d8

📥 Commits

Reviewing files that changed from the base of the PR and between 9184a0b and 229d466.

📒 Files selected for processing (1)

• .github/workflows/build.yml

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Walkthrough

The pull request updates the GitHub Actions workflow configuration in .github/workflows/build.yml. Specifically, the check-skip job's runs-on field is modified to use a new variable RUNNER_CHECK_SKIP with a fallback to ubuntu-24.04-arm, replacing the previous configuration that used RUNNER_AMD64 with a fallback to ubuntu-24.04. This change determines which runner environment executes the bootstrap/decision step, while all downstream usages of the job's outputs remain unaffected.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main change: moving the check-skip CI job to a cheaper ARM runner with specific focus on bootstrap optimization.
Description check	✅ Passed	The description is well-organized, clearly related to the changeset, and explains the problem, solution, and validation steps for the runner configuration change.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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