fix(release): retry transient snapshot download failures with backoff

Both package-closure matrix jobs died 25 minutes in when
snapshot.ubuntu.com answered 500/502 during the repeat build. Retry
server and network failures up to three times with backoff, matching
the curl fetchers' --retry 3. Client errors still fail immediately,
policy violations are never retried, and every download remains
digest-verified, so retries cannot alter the closure.

Co-authored-by: Cursor <cursoragent@cursor.com>