Update staging.yaml

.github/workflows/staging.yaml

@@ -13,6 +13,9 @@ jobs:
         project_id: ${{ secrets.GCP_PROJECT_ID }}
         service_account_key: ${{ secrets.GCP_SA_KEY }}
         export_default_credentials: true
+    - name: Install kubectl
+      run: |
+        sudo apt-get install kubectl
     - name: Log in to gcloud CLI
       run: gcloud auth configure-docker
     - name: Checkout