fix(ci): count the kubeconfig's clusters correctly

The cluster-count guard I added used `grep -c '"name"'`, which counts
LINES.  The jsonpath output is a single line, so the count was 1 for a
kubeconfig with clusters and 1 for one without - which made the guard
reject every kubeconfig:

    ERROR: the kubeconfig has no cluster entries.

The count is now done with `grep -o . | wc -l`, which counts occurrences
rather than lines.  Tested against both cases:

- a real kubeconfig with a stale CA -> the CA is refreshed and kubectl works
- a kubeconfig with no clusters        -> refuses with a clear message

scripts/kube-ca-refresh.sh

@@ -56,8 +56,10 @@ if [ -z "$EXPECTED" ]; then
 fi
 
 # Guard against a kubeconfig with no clusters, or an empty one.
-CLUSTER_COUNT=$(kubectl config view --raw -o jsonpath='{.clusters}' 2>/dev/null \
-    | grep -c '"name"' || true)
+# `grep -c` counts LINES, and the jsonpath output is a single line, so the
+# count is done with grep -o and wc -l instead.
+CLUSTER_COUNT=$(kubectl config view --raw -o jsonpath='{.clusters[*].name}' 2>/dev/null \
+    | grep -o . | wc -l || true)
 if [ "${CLUSTER_COUNT:-0}" -eq 0 ]; then
     echo "ERROR: the kubeconfig has no cluster entries." >&2
     echo "  The KUBEC_CONFIG_BUILDX_NEW_2 secret looks wrong or empty." >&2