The multi-arch publish job fails with 'extern location does not exist'
errors for version_check when BuildKit cache mounts contain stale
artifacts. The existing retry logic (cargo clean + retry) was
ineffective because:
1. cargo clean relies on its own metadata to decide what to remove,
which can itself be stale on BuildKit cache mounts shared across
different build profiles (debug vs release).
2. sccache remained enabled during the retry, so a corrupt memcached
cache entry would reproduce the same failure even after cleaning.
Replace cargo clean with rm -rf /build/target/* for a thorough wipe,
and unset RUSTC_WRAPPER on retry so cargo falls back to raw rustc.
John T. Myers