roachtest: lock backup fixtures from GC after failed restore

[kev-cao]

If a restore from our backup fixtures fail in a roachtest, we should flag the fixture in question to prevent it from being GC'd. This gives us a bigger window of time to investigate restore failures before the backup is cleaned up after the next fixture is generated.

Jira issue: CRDB-58143

[blathers-crl]

cc @cockroachdb/disaster-recovery