[CI] Do not run Trivy scan by default

To get around recent TOOMANYREQUESTS from Trivy during DB download,
this commit does the following:

- removes the scan from ci-pr.yml altogether
- makes scan optional (default false) for ci.yml
- schedules scan to run every 3 days at 14:00 UTC

.github/workflows/ci-pr.yml

@@ -105,14 +105,9 @@ jobs:
               -PspringBootVersion="$BOOT_VERSION" \
               -PsampleTests \
               :runAllSampleTests
-  scan:
-    needs: [prerequisites]
-    if: needs.prerequisites.outputs.runjobs
-    uses: ./.github/workflows/trivy-scan.yml
-
   done:
     runs-on: ubuntu-latest
-    needs: [ build_and_verify, check_samples, scan ]
+    needs: [ build_and_verify, check_samples ]
     steps:
       - name: 'Done'
         shell: bash