ci: install kubectl in deploy job for ARC self-hosted runners

ARC runner image doesn't include kubectl. Install it via curl
before the deploy step so rollout restarts work correctly.

.github/workflows/build-lux.yml

@@ -177,6 +177,11 @@ jobs:
         with:
           token: ${{ secrets.DIGITALOCEAN_ACCESS_TOKEN }}
 
+      - name: Install kubectl
+        run: |
+          curl -LO "https://dl.k8s.io/release/$(curl -L -s https://dl.k8s.io/release/stable.txt)/bin/linux/amd64/kubectl"
+          chmod +x kubectl && mv kubectl /usr/local/bin/kubectl
+
       - name: Configure kubectl
         run: doctl kubernetes cluster kubeconfig save lux-k8s