The delete branch required Datastore.Allocate (or was hitting a
privilege-separated token ACL edge case) and produced 403s on re-runs
against the same commit SHA. Since the ISO bytes are reproducible for
a given SHA — furtka-<sha>.iso is content-addressed — we can just
reuse whatever is already in PVE storage instead of cycling it.
Fixes the "runs-on-same-sha" re-dispatch case without needing any extra
PVE permission, and shaves ~2 min off repeated smoke runs.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>