script checks: use cat instead of ls for exit code agreement

e2e/consul/input/checks_group.nomad

@@ -45,7 +45,7 @@ job "group_check" {
         task     = "test"
         interval = "2s"
         timeout  = "2s"
-        command  = "ls"
+        command  = "cat"
         args     = ["${NOMAD_TASK_DIR}/alive-2b"]
       }
     }

e2e/consul/input/checks_task.nomad

@@ -39,7 +39,7 @@ job "task_check" {
           type     = "script"
           interval = "2s"
           timeout  = "2s"
-          command  = "ls"
+          command  = "cat"
           args     = ["${NOMAD_TASK_DIR}/alive-2b"]
         }
       }