chore: standardize GitHub Actions runs-on expression

[chore-bot]

Why

This change prepares this repository for the migration from the old self-hosted GitHub Actions runners to the new ARCv2-managed self-hosted runners.

Using the standard runner variables lets Platform switch workloads over centrally while still allowing repository or team-level overrides where needed. Copilot workflows use their own runner override so they can move independently of the general build fleet.

Summary

• standardize GitHub Actions runs-on entries to ${{ vars.GLOBAL_RUNNER_OVERWRITE || vars.TEAM_RUNNER_LABEL || 'boxed-runner' }}
• use ${{ vars.COPILOT_RUNNER_OVERWRITE || 'copilot-code-review-runner' }} for Copilot workflow files
• pin github-actions-artifact-publishing/publish-artifact to 308fab83c075d899591843c2d66a969067899e2b
• pin invalid github-actions-backend/build-and-test refs to cc09821c2cbdfb70e79e1fc970a3d457589d55a9

Impact

• no application code changes
• workflow jobs continue to run on self-hosted runners, but through the ARCv2-compatible labels/overrides
• action pins are updated where required so the workflows use versions that support the ARCv2 migration