swarm-bao-otel-oidc still reads the secret from bao under the collector's
own leaf, but now writes it to /run/swarm-otel-oidc on the host (tmpfs),
which the container bind-mounts read-only for the collector's
LoadCredential. Before, it wrote it to
/var/lib/nixos-containers/swarm-otel/var/lib/swarm-otel-oidc/<id>.secret on
persistent disk, where a superseded secret stayed after rotation and went
into backups of the container root. Host tmpfiles now removes that old file.
Rotation is unchanged: the ten-minute timer rewrites the file when the value
differs and refresh_consumer restarts the collector. After a reboot the
secret is fetched again; until the store answers the collector starts on an
empty file and the stores refuse its pushes and scrapes.
Part of #4998 (slice 2a).
7539ef82f7