ci: correct the matrix runner comment's invariant

[owine]

Follow-up to #110, reconciling this repo's comment with owine/renovate-config 24ad0fd.

#110 left a comment telling readers to keep a runner: ubuntu-<major>.<minor> shape. That is wrong on two counts: it does not describe the arm leg (ubuntu-26.04-arm), and it implies the suffix sits outside what Renovate tracks.

Verified in lib/modules/datasource/github-runners/index.ts on renovate main — the ubuntu release list contains 26.04 and 26.04-arm as distinct versions. The preset's capture takes the whole remainder after the OS name, which is correct and mirrors the built-in manager's own regex, so both halves land in one PR.

Confirmed the preset's pattern extracts this repo's two legs as ubuntu/26.04 and ubuntu/26.04-arm.

Comment-only; no behaviour change.

Summary by Sourcery

Enhancements:

• Correct the GitHub Actions runner matrix guidance to document independent tracking of x64 and ARM Ubuntu runner versions.

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[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Updates a workflow-only comment to accurately describe how the shared github-runners manager extracts complete runner versions, including the -arm suffix; behavior is unchanged.

File-Level Changes

Change	Details	Files
Correct the workflow comment to document the shared github-runners manager and its full-version invariant.	Replace the outdated custom-manager name and add the referenced preset revision. Document the required single-line runner: <os>-<version> syntax, including quoted and unquoted forms. Clarify that 26.04-arm is tracked as a distinct datasource version and that the x64 and arm legs may update independently.	.github/workflows/build.yml

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