The workflow owns its triggers and ci.yml does not call it. A
reusable-workflow call holds the caller run in progress for the full
build, and GitHub refuses `gh run rerun` on a run that is still in
progress. A separate run reruns and cancels on its own.
The job restores /nix/store from the GitHub Actions cache and saves from
main only. A cache that a PR writes is visible to that PR alone, so a
save there spends the quota of the repository and helps no later run.
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