Actions(trufflehog): Fix for external PRs (#108215)

.github/workflows/trufflehog.yml

@@ -21,15 +21,17 @@ jobs:
       contents: read # clone the repo
 
     steps:
+      - name: Calculate fetch depth
+        id: fetch_depth
+        shell: bash
+        run: echo "fetch_depth=$(( ${{ github.event.pull_request.commits }} + 2 ))" >> "$GITHUB_OUTPUT"
       - name: Checkout code
         uses: actions/checkout@v4
         with:
           persist-credentials: false
-          fetch-depth: 0
-          ref: ${{ github.head_ref }}
+          fetch-depth: ${{ steps.fetch_depth.outputs.fetch_depth }}
+          ref: ${{ github.event.pull_request.head.ref }}
       - name: Trufflehog
         uses: trufflesecurity/trufflehog@eafb8c5f6a06175141c27f17bcc17941853d0047 # v3.90.0
         with:
-          base: ${{ github.event.pull_request.base.sha }}
-          head: ${{ github.event.pull_request.head.sha }}
           extra_args: --results=verified