[stable/insights-agent] Add more options for Trivy

[rbren]

Why This PR? a short description of why this PR is needed

This PR adds some additional options to Trivy, particularly to allow it to operate in offline mode.

Checklist:

• [x] I have included the name of the chart in the title of this PR in square brackets i.e. [stable/goldilocks].
• [x] I have updated the chart version in Chart.yaml following Semantic Versioning.
• [x] Any new values are backwards compatible and/or have sensible default.
• [x] Any new values have been added to the README for the Chart, or helm-docs --sort-values-order=file has been run for the charts that support it.

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