Fix unbound setup for ubuntu-noble

dns-root-data has been demoted to a "Recommends" dependency of
unbound, which we don't install. Sadly the default unbound
configuration is broken without it.

Change-Id: Ie285d1c058c4ad7c6579f25cf24884d8e396e1dc

nodepool/elements/nodepool-base/pkg-map

@@ -4,6 +4,11 @@
       "bookworm": {
         "unbound": "unbound dns-root-data"
       }
+    },
+    "ubuntu": {
+      "noble": {
+        "unbound": "unbound dns-root-data"
+      }
     }
   }
 }