Recipes: make cwd default to slave_build

scripts/slave/recipes/skia/autoroll.expected/AutoRoll_failed.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "config",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/chromium/src.git'}]"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient setup"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "sync",
       "--nohooks",
       "--force",
       "--verbose",
       "--delete_unversioned_trees",
       "--output-json",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient sync",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"solutions\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"src/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": 170242@@@",
       "@@@STEP_LOG_LINE@json.output@    }, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"src/third_party/skia/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": 143121@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
@@ skipping 26 matching lines @@
     "name": "git config user.email"
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport urllib2\nimport sys\nimport time\n\nattempts = 5\nres = None\nfor attempt in range(attempts):\n  try:\n    res = urllib2.urlopen(sys.argv[1]).read()\n    break\n  except urllib2.URLError:\n    if attempt == attempts - 1:\n      raise\n    time.sleep(2 ** attempt)\nwith open(sys.argv[2], 'w') as f:\n  f.write(res)\n",
       "http://skia-tree-status.appspot.com/arb_is_stopped",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "is_stopped",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"is_stopped\": false@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import urllib2@@@",
       "@@@STEP_LOG_LINE@python.inline@import sys@@@",
       "@@@STEP_LOG_LINE@python.inline@import time@@@",
@@ skipping 28 matching lines @@
       "step returned non-zero exit code: 1",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$final_result",
     "reason": "Step('do auto_roll') failed with return_code 1",
     "status_code": 1
   }
 ]