Instruct trivy to use fallback for downloading db.

[purple4reina]

Seeing error when running nightly vulnerability scan

Running Trivy with options: trivy image public.ecr.aws/datadog/lambda-extension:latest
2024-10-20T01:32:03Z	INFO	[vulndb] Need to update DB
2024-10-20T01:32:03Z	INFO	[vulndb] Downloading vulnerability DB...
2024-10-20T01:32:03Z	INFO	[vulndb] Downloading artifact...	repo="ghcr.io/aquasecurity/trivy-db:2"
2024-10-20T01:32:04Z	ERROR	[vulndb] Failed to download artifact	repo="ghcr.io/aquasecurity/trivy-db:2" err="OCI repository error: 1 error occurred:\n\t* GET https://ghcr.io/v2/aquasecurity/trivy-db/manifests/2: TOOMANYREQUESTS: retry-after: 1.084625ms, allowed: 44000/minute\n\n"
2024-10-20T01:32:04Z	FATAL	Fatal error	init error: DB error: failed to download vulnerability DB: OCI artifact error: failed to download vulnerability DB: failed to download artifact from any source

Using solution from this github issue, adds fallback db when rate limit is hit.

[purple4reina]

Tested this branch out, retry logic works