hotfix: ensure that schema is compatible with tutorial VM config

We added a hotfix to releases/v0.19 with a feature flag, but the flag
is incompatible with the config schema on `develop`.

- [x] Ensure schema is compatible on develop even though config option is unused.

lib/spack/spack/schema/concretizer.py

@@ -15,6 +15,7 @@
         "additionalProperties": False,
         "properties": {
             "reuse": {"type": "boolean"},
+            "enable_node_namespace": {"type": "boolean"},
             "targets": {
                 "type": "object",
                 "properties": {