Let a failed image build leave the rootfs that explains it

[rldyourmnd]

Issue #265 has three failed integration-image publishes and two distinct errors:

publish immutable worker image: incus command failed: exit status 1:
  Error: websocket: close 1006 (abnormal closure): unexpected EOF
  Error: readdirent /var/lib/incus/containers/gha-fleet_gha-image-builder-.../
         rootfs/usr/lib/python3/dist-packages/botocore-1.34.46.egg-info:
         bad file descriptor

Both name a path inside the builder's rootfs, and the orchestrator's cleanup defer deletes that rootfs on the way out. So every attempt destroyed the only artifact that could explain it, and two sessions have now reasoned about the failure from the message alone. The issue's own suggested next step asks for exactly this: "reproduce with incus publish run directly against a preserved builder, so the container survives the failure and the rootfs can be inspected."

--preserve-failed-builder is opt-in, so the default still deletes and a routine failure cannot strand a member.

It is never silent. The preserved case wraps the original error with the instance name, the member it sits on, and the command that removes it, because a preserved builder holds disk and an image-maintenance capacity lease until someone does:

publish immutable worker image: ... (builder "gha-image-builder-..." preserved
on gha-runner-1 for inspection; it holds disk and a capacity lease until
`incus delete --project gha-fleet gha-image-builder-... --force`)

The underlying error is wrapped, not replaced — errors.Is still finds it, so preserving cannot hide the reason the builder is worth inspecting.

One test drives a build to a failing publish twice, with and without the flag, and asserts: the default still deletes and does not claim to preserve; the flag keeps the instance, wraps rather than discards the cause, and names all three of instance, member and removal command. It fails on the rollback of the flag.

make verify green.

https://claude.ai/code/session_01NpzpgiRaxi5mHVTMoRYndt