[Issue #2496] cache trivy (#2839)

## Summary

Very nearly fixes #2496

### Time to review: __1 mins__

## Context for reviewers

I'm not committing to this 100% solving #2496 because the daily cronjob
can fail (on trivy rate limits) which would just mean we are back in the
same situation we have always been in. So I want to add tries on this
stuff because I can call the issue fully solved.

90% of the meat is this PR is pulled from Trivy's docs, I don't know
anything about the `oras` stuff

.github/workflows/vulnerability-scans.yml

@@ -94,6 +94,48 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Get current date
+        id: date
+        run: echo "date=$(date +'%Y-%m-%d')" >> "$GITHUB_OUTPUT"
+
+      - name: Restore cached trivy vulnerability and Java DBs
+        id: trivy-cache
+        uses: actions/cache/restore@v4
+        with:
+          path: ${{ github.workspace }}/.cache/trivy
+          key: trivy-cache-${{ steps.date.outputs.date }}
+
+      # Download and extract the vulnerability DB and Java DB
+      # This is based on the instructions here:
+      # https://github.com/aquasecurity/trivy-action/?tab=readme-ov-file#updating-caches-in-the-default-branch
+
+      - name: Setup oras
+        if: steps.trivy-cache.outputs.cache-hit != 'true'
+        uses: oras-project/setup-oras@v1
+
+      - name: Download and extract the vulnerability DB
+        if: steps.trivy-cache.outputs.cache-hit != 'true'
+        run: |
+          mkdir -p "$GITHUB_WORKSPACE/.cache/trivy/db"
+          oras pull ghcr.io/aquasecurity/trivy-db:2
+          tar -xzf db.tar.gz -C "$GITHUB_WORKSPACE/.cache/trivy/db"
+          rm db.tar.gz
+
+      - name: Download and extract the Java DB
+        if: steps.trivy-cache.outputs.cache-hit != 'true'
+        run: |
+          mkdir -p "$GITHUB_WORKSPACE/.cache/trivy/java-db"
+          oras pull ghcr.io/aquasecurity/trivy-java-db:1
+          tar -xzf javadb.tar.gz -C "$GITHUB_WORKSPACE/.cache/trivy/java-db"
+          rm javadb.tar.gz
+
+      - name: Cache DBs
+        if: steps.trivy-cache.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v4
+        with:
+          path: ${{ github.workspace }}/.cache/trivy
+          key: trivy-cache-${{ steps.date.outputs.date }}
+
       - name: Restore cached Docker image
         uses: actions/cache/restore@v4
         with:
@@ -116,6 +158,9 @@ jobs:
           ignore-unfixed: true
           vuln-type: os
           scanners: vuln,secret
+        env:
+          TRIVY_SKIP_DB_UPDATE: true
+          TRIVY_SKIP_JAVA_DB_UPDATE: true
 
       - name: Save output to workflow summary
         if: always() # Runs even if there is a failure