Substitute cluster env variable

clusters/testing/flux-system/kustomization.yaml

@@ -3,3 +3,13 @@ kind: Kustomization
 resources:
   - gotk-components.yaml
   - gotk-sync.yaml
+patches:
+  - patch: |
+      - op: add
+        path: /spec/postBuild
+        value:
+          substitute:
+            cluster_env: staging
+    target:
+      kind: Kustomization
+      name: flux-system