ci: add Trivy fallback DB repositories

[adamconnelly]

Description of the change

Just adding these now to try to avoid people hitting the Trivy rate limiting issues later.

Type of change

• Bug fix (non-breaking change that fixes an issue);
• New feature (non-breaking change that adds functionality);
• Breaking change (fix or feature that would cause existing functionality not to work as expected);
• Other (miscellaneous, GitHub workflow changes, changes to the PR template);

Checklists

Development

• Lint rules pass locally;
• All tests related to the changed code pass in development;

Code review

• This pull request has a descriptive title and sufficient context for a reviewer. There may be a screenshot or screencast attached;
• This pull request is no longer a draft;

Deployment

• Selected merge strategy is squash merge;
• Changes have been reviewed by at least one other engineer;