[mcocdawc]

Runs the HPC legs on a pull request only once it carries ready-for-hpc (takes effect on the next push or Re-run all jobs), and reports them under their own status ci-result/hpc-atos instead of ci-result, so they stay required before merge. Needs ecmwf/ci-infrastructure#210; branched from #663, whose commits show here until it merges.

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