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

recipe_modules/step/example.expected/infra_failure.json

Index: recipe_modules/step/example.expected/infra_failure.json
diff --git a/recipe_modules/step/example.expected/infra_failure.json b/recipe_modules/step/example.expected/infra_failure.json
index 0865b5cdecc6d75999fabcbc2ee1c9be1a5d7595..4ad5702b5383345211315a4fe2bfb79dae94569c 100644
--- a/recipe_modules/step/example.expected/infra_failure.json
+++ b/recipe_modules/step/example.expected/infra_failure.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."