[step_runner] run _merge_envs in simulation too.

recipe_modules/step/tests/inject_paths.expected/basic.json

 [
   {
     "cmd": [
       "echo",
       "hi"
     ],
     "env": {
       "FOO": "bar"
     },
     "name": "test step (no env)"
   },
   {
     "cmd": [
       "echo",
       "hi"
     ],
     "env": {
       "PATH": "something"
     },
     "name": "test step (env)"
   },
   {
     "cmd": [
       "echo",
       "hi"
     ],
     "env": {
-      "PATH": "something:%(PATH)s"
+      "PATH": "something:<PATH>"
     },
     "name": "test step (env, $PATH)"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]