fix: Use postRenderer to replace broken kuberlr-kubectl image in rancher-backup hook

The chart's post-install hook hardcodes rancher/kuberlr-kubectl which
can't download kubectl. Use Flux postRenderers to patch the job image
to bitnami/kubectl at render time.

infrastructure/addons/rancher-backup/helmrelease-rancher-backup.yaml

@@ -20,11 +20,9 @@ spec:
     createNamespace: true
     remediation:
       retries: 3
-    skipHooks: true
   upgrade:
     remediation:
       retries: 3
-    skipHooks: true
   values:
     image:
       repository: rancher/backup-restore-operator
@@ -32,3 +30,13 @@ spec:
       image:
         repository: bitnami/kubectl
         tag: "1.34"
+  postRenderers:
+    - kustomize:
+        patches:
+          - target:
+              kind: Job
+              name: rancher-backup-patch-sa
+            patch: |
+              - op: replace
+                path: /spec/template/spec/containers/0/image
+                value: bitnami/kubectl:1.34