Fix /trivy action running against target branch instead of PR branch

Signed-off-by: Brad Davidson <brad.davidson@rancher.com>

.github/workflows/trivy.yaml

@@ -16,7 +16,7 @@ jobs:
     - name: Checkout PR code
       uses: actions/checkout@v4
       with:
-        ref: ${{ github.event.issue.pull_request.head.ref }}
+        ref: refs/pull/${{ github.event.issue.number }}/head
     
     - name: Comment Status on PR
       run: |