fix kind get clusters via nerdctl

Now raising errors when using nerdctl for the provider

```console
> ./kind --version
kind version 0.26.0-alpha
> nerdctl --version
nerdctl version 2.0.1
> ./kind get clusters
ERROR: failed to list clusters: command "nerdctl ps -a --filter label=io.x-k8s.kind.cluster --format '{{index .Labels "io.x-k8s.kind.cluster"}}'" failed with error: exit status 1
Command Output: time="2024-12-09T18:01:03+09:00" level=fatal msg="template: :1:2: executing \"\" at <index .Labels \"io.x-k8s.kind.cluster\">: error calling index: cannot index slice/array with type string"
```

nerdctl fixed the `.Label` behavior in v1.7.0.
2af4cef9e7
However `index .Labels` syntax is not yet supported at least in v2.0.1.
(The style is also used for podman provider, and it is available)

This commit follows up https://github.com/kubernetes-sigs/kind/pull/3429

Signed-off-by: Kenichi Kamiya <kachick1@gmail.com>

pkg/cluster/internal/providers/nerdctl/provider.go

@@ -125,7 +125,7 @@ func (p *provider) ListClusters() ([]string, error) {
 		// filter for nodes with the cluster label
 		"--filter", "label="+clusterLabelKey,
 		// format to include the cluster name
-		"--format", fmt.Sprintf(`{{index .Labels "%s"}}`, clusterLabelKey),
+		"--format", fmt.Sprintf(`{{.Label "%s"}}`, clusterLabelKey),
 	)
 	lines, err := exec.OutputLines(cmd)
 	if err != nil {