workload-cluster: fix namespaces of the GitRepository used by cattle-agent-xxx Kustomization

For a workload cluster, the GitRepository created for the cattle-agent-<xxxx> Kustomization by capi-rancher-import needs to be the one from the workload cluster namespace, not sylva-system.

Having one instead of the other is not a problem in practice today, because the files used in capi-rancher-import GitRepository by the cattle-agent-<xxxx> Kustomization has never evolved (https://gitlab.com/sylva-projects/sylva-elements/helm-charts/capi-rancher-import/-/tree/main/cattle-kustomize?ref_type=heads).

So this is a very minor fix.