swarm-bao-ci-runner still reads the token from bao under the runner's own
ci-runner leaf, but now writes it to /run/swarm-ci-runner on the host
(tmpfs), which the container bind-mounts read-only for the register
unit's LoadCredential. Before, it wrote the token to
/var/lib/nixos-containers/swarm-ci-runner/var/lib/swarm-ci-runner/registration-token
on persistent disk, where a superseded token stayed after rotation and
went into backups of the container root. The container's tmpfiles now
remove that old file.
Rotation is unchanged: the ten-minute timer rewrites the file when the
value differs and refresh_consumer restarts the runner. After a reboot
the token is fetched again; until the store answers, a runner that is
already registered keeps running and only a new registration waits.
Part of #4998 (slice 1).
23807c5d0c