A request on $SWARM.builder.lock-meta carries a rendered flake.nix. The
builder writes it into a scratch directory, runs `nix flake lock` there
with the same forge credential helper its builds use, answers with the
flake.lock nix wrote, and removes the directory. It commits nothing; the
controller commits the lock beside the flake.
Locks are answered in their own loop, like archetype evaluations, so one
does not wait behind an hour-long build. The auth callout grants the
subject to the builder (subscribe) and the controller (publish) only.
Ported from c574a1694 onto the swarm-wire crate.
Refs #4857
