ci: labels no longer re-run CI; runner recipe in .ci/runner/

[mcocdawc]

Description

• ci.yml no longer runs on labeled, so adding a run-downstream-ci:* label no longer re-runs CI; with Downstream CI starts on its label, without re-running CI ci-infrastructure#104 the label starts the downstream CI directly. The regenerate bot brings trigger-downstream*.yml up to date.
• The runner recipe moves from .ci/build.sh.j2 to .ci/runner/build.sh.j2, next to .ci/hpc/build.sh.j2 (folded in from ci: the runner recipe lives in .ci/runner/, next to .ci/hpc/ #188). Only the manifest path changes; rendered scripts and generated workflows are unchanged.

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• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.