ci(trivy): fix scan failures due to TOOMANYREQUESTS DB error

[aldousalvarez]

Commit to be reviewed

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ci(trivy): fix scan failures due to TOOMANYREQUESTS DB error

Primary Changes
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1. Updated the ci.yaml to make the trivy scan 
   run only on the weekends

Fixes #3652

Pull Request Requirements

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A Must Read for Beginners
For rebasing and squashing, here's a must read guide for beginners.

[outSH]

Looks fine, but I'd rather have a separate "weekly" job triggered by cron (https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#schedule), it will be easier to see and add new weekly jobs, and it doesn't convolute our CI job with more prerequisites. @petermetz any thoughts?