fix(db): Dowload database when missing but metadata still exists

[tom1299]

Description

In the case where the trivy database has been deleted, but the metadata.json is still there and the database has not yet expired, the database is not downloaded.
Steps to reproduce:

trivy -d --cache-dir /tmp/trivy-cache image --download-db-only --db-repository ghcr.io/aquasecurity/trivy-db
rm /tmp/trivy-cache/db/trivy.db
trivy -d --cache-dir /tmp/trivy-cache image --download-db-only --db-repository ghcr.io/aquasecurity/trivy-db

This PR contains a change so that the database will be downloaded if trivy.db is missing even if the metadata is correct.
See also discussion here.

I could also change the implementation so that only a warning would be logged. That way the existing behaviour of the function would remain.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[DmitriyLewen]

Thanks for your contribution!