recipe engine: extract infra-specific paths out of the engine

recipe_modules/python/example.expected/basic.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "import sys; sys.exit(0)"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "success",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@success@This step is a success@@@",
       "@@@STEP_LOG_END@success@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "import sys; sys.exit(1)"
     ],
-    "cwd": "[SLAVE_BUILD]",
     "name": "failure",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_TEXT@This step is a failure :(@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]