archive-staging-rke2: Add missing ingress namespace

Ref. swh/infra/sysadm-environment#4699

archive-staging-rke2/namespaces.yaml

@@ -31,3 +31,9 @@ apiVersion: v1
 kind: Namespace
 metadata:
   name: cert-manager
+---
+# Namespace dedicated to the ingress-nginx environment
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: ingress-nginx