fix: validate workflow

.github/workflows/validate-k8s.yml

@@ -36,6 +36,13 @@ jobs:
         with:
           version: 'v3.9.0'
 
+      - name: Install Helm Diff plugin
+        run: |
+          if ! helm plugin list | grep -q '^diff'; then
+            helm plugin install https://github.com/databus23/helm-diff
+          fi
+          helm diff version
+
       - name: Determine DNS_HOST based on environment
         id: set_dns
         env: