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

recipe_modules/step/example.expected/invalid_access.json

 [
   {
     "cmd": [
       "echo",
       "Hello World"
     ],
     "name": "hello"
   },
   {
     "cmd": [
       "echo",
       "Why hello, there."
     ],
     "name": "hello (2)"
   },
   {
     "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, $friend."
     ],
     "env": {
       "friend": "Darth Vader"
     },
     "name": "goodbye"
   },
   {
@@ skipping 92 matching lines @@
       "nothing"
     ],
     "name": "no-op"
   },
   {
     "name": "$result",
     "reason": "Invalid Step Data Access: StepDataAttributeError('The recipe attempted to access missing step data \"json\" for step \"no-op\". Please examine that step for errors.',)",
     "status_code": -1
   }
 ]