[recipe_modules/step] do not set cwd if it is start_dir.

recipe_modules/step/example.expected/invalid_access.json

Index: recipe_modules/step/example.expected/invalid_access.json
diff --git a/recipe_modules/step/example.expected/invalid_access.json b/recipe_modules/step/example.expected/invalid_access.json
index 82887f19a1a13d4e77bdabd8deb81de4e5c0a516..ad249715e34a087a7b38550f5456ea444fb6b606 100644
--- a/recipe_modules/step/example.expected/invalid_access.json
+++ b/recipe_modules/step/example.expected/invalid_access.json
@@ -15,6 +15,30 @@
   },
   {
     "cmd": [
+      "mkdir",
+      "something"
+    ],
+    "name": "mk subdir"
+  },
+  {
+    "cmd": [
+      "bash",
+      "-c",
+      "echo Why hello, there, in a subdir."
+    ],
+    "cwd": "[START_DIR]/something",
+    "name": "something"
+  },
+  {
+    "cmd": [
+      "bash",
+      "-c",
+      "echo what happen"
+    ],
+    "name": "start_dir ignored"
+  },
+  {
+    "cmd": [
       "bash",
       "-c",
       "echo Good bye, $friend."