ci: Trivy enhancements

[khvn26]

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Changes

This moves Trivy database caching from the PR workflow to a new cron-based workflow, which also runs Trivy against main tags every three hours.

This fixes CI for external contributions and makes sure we have up-to-date security reports for current main.

How did you test this code?

This is a CI change.

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)	Visit Preview		Oct 8, 2024 2:47pm
flagsmith-frontend-preview	⬜️ Ignored (Inspect)	Visit Preview		Oct 8, 2024 2:47pm
flagsmith-frontend-staging	⬜️ Ignored (Inspect)	Visit Preview		Oct 8, 2024 2:47pm

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4703	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4703	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-4703	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-4703	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4703	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4703	Finished ✅	Results ✅

[github-actions]

Uffizzi Preview deployment-57025 was deleted.

[github-advanced-security]

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[matthewelwell]

I think the name here needs updating?

[khvn26]

Done.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.21%. Comparing base (8e6b241) to head (75f7529).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4703   +/-   ##
=======================================
  Coverage   97.21%   97.21%           
=======================================
  Files        1171     1171           
  Lines       40651    40651           
=======================================
  Hits        39517    39517           
  Misses       1134     1134           

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