.Values._internal.is_upgrade broken on workload-clusters

@feleouet analyzed a failure on !3101 (merged) by @cristian.manda and observed that .Values._internal.is_upgrade is never true for workload clusters.

The reason is that this boolean is based on a test of whether the sylva-units-status ConfigMap exists (in the ns of the sylva-units Helm release, in the mgmt cluster), but for workload clusters this ConfigMap is (unfortunately) not created in the right cluster (it's created in the workload cluster!) and not in the right namespace.