rpk: stop using args[0] in cloud cluster select

There might be cases where the user is using
rpk cloud cluster select in a single-cluster org
and args will be null, causing a panic.

Fixes #18270

src/go/rpk/pkg/cli/cloud/cluster/select.go

@@ -60,10 +60,10 @@ an existing self-hosted profile, please rename that profile or use the
 Either:
     rpk profile select %[1]q
     rpk profile rename-to $something_else
-    rpk cloud cluster select %[2]q
+    rpk cloud cluster select [NAME]
 Or:
-    rpk cloud cluster select %[2]q --profile $another_something
-`, ee.Name, args[0])
+    rpk cloud cluster select [NAME] --profile $another_something
+`, ee.Name)
 				os.Exit(1)
 			}
 			out.MaybeDieErr(err)