provider/tekton: when pruning is configured and old builds are pruned, log lookup doesn't work

For example, like this: https://tangled.org/strings/did:plc:e5nncb3dr5thdkjir5cfaqfe/3ml52ngsafs22

To replicate:

1. Set up Tack in Kubernetes with Tekton
2. Install the Tekton pruner: https://tekton.dev/docs/pruner/
3. Run a CI job
4. The pruner nukes it after 6 minutes
5. Look at the CI logs
6. Infinite tack death spiral

Maybe tack needs to store the logs to handle this?