Recipes: make cwd default to slave_build

scripts/slave/recipe_modules/bot_update/example.expected/tryjob_fail_patch.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_rel",
       "--slave",
       "totallyaslave-c4",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'svn://svn.chromium.org/chrome/trunk/src'}]",
       "--root",
       "src",
       "--revision_mapping_file",
       "{}",
       "--patch_url",
       "http://src.chromium.org/foo/bar",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "bot_update",
     "~followup_annotations": [
       "@@@STEP_TEXT@Some step text@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"did_run\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"log_lines\": [@@@",
       "@@@STEP_LOG_LINE@json.output@    [@@@",
       "@@@STEP_LOG_LINE@json.output@      \"patch error\", @@@",
       "@@@STEP_LOG_LINE@json.output@      \"Patch failed to apply\"@@@",
       "@@@STEP_LOG_LINE@json.output@    ]@@@",
       "@@@STEP_LOG_LINE@json.output@  ], @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_failure\": true, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_root\": \"src\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {}, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"root\": \"src\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"step_text\": \"Some step text\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LOG_LINE@patch error@Patch failed to apply@@@",
       "@@@STEP_LOG_END@patch error@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "import sys; sys.exit(1)"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "Patch failure",
     "~followup_annotations": [
       "step returned non-zero exit code: 1",
       "@@@STEP_TEXT@Check the bot_update step for details@@@",
       "@@@STEP_FAILURE@@@"
     ]
   },
   {
     "name": "$final_result",
     "reason": "Step('Patch failure') failed with return_code 1",
     "status_code": 1
   }
 ]