mirror.gcr.io intermittently fails DNS resolution in CI runner environment.
Add --db-repository ghcr.io/aquasecurity/trivy-db to all three Trivy scan steps
as a reliable alternative to the default GCR mirror.
Fixes pipeline #1056 failure (security-trivy-api/orchestrator/web all failed
with: 'dial tcp: lookup mirror.gcr.io on 127.0.0.11:53: server misbehaving')