[step_runner] run _merge_envs in simulation too.

recipe_modules/step/examples/full.expected/exceptional.json

 [
   {
     "cmd": [
       "echo",
       "Hello World"
     ],
     "name": "hello"
   },
   {
     "cmd": [
@@ skipping 37 matching lines @@
       "friend": "Darth Vader"
     },
     "name": "goodbye"
   },
   {
     "cmd": [
       "recipes.py",
       "--help"
     ],
     "env": {
-      "PATH": "RECIPE_PACKAGE_REPO[recipe_engine]:%(PATH)s"
+      "PATH": "RECIPE_PACKAGE_REPO[recipe_engine]:<PATH>"
     },
     "name": "recipes help"
   },
   {
     "cmd": [
       "bash",
       "-c",
       "exit 3"
     ],
     "name": "anything is cool"
@@ skipping 29 matching lines @@
       "step returned non-zero exit code: 1",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "Uncaught Exception: ValueError('goodbye must exit 0!',)",
     "status_code": -1
   }
 ]