add cert-manager namespace so we can make the cert-manager override before installing it

.gitlab-ci.yml

@@ -331,6 +331,7 @@ setup-stackspin:
     - kubectl create namespace flux-system
     - kubectl apply -k ${CLUSTER_DIR}
     # Add an override so cert-manager uses the ZeroSSL ClusterIssuer
+    - kubectl create namespace cert-manager
     - kubectl apply -n cert-manager -f ./install/overrides/stackspin-cert-manager-override.yaml
     # Install flux and general, non-app specific secrets
     - bash ./install/install-stackspin.sh