Recipes: make cwd default to slave_build

scripts/slave/recipes/run_presubmit.expected/nacl.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "chromium_presubmit",
       "--slave",
       "TestSlavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'native_client', 'url': 'https://chromium.googlesource.com/native_client/src/native_client.git'}]",
       "--root",
       "native_client",
       "--revision_mapping_file",
       "{\"native_client\": \"got_revision\"}",
       "--issue",
       "12853011",
       "--patchset",
       "1",
       "--rietveld_server",
       "https://codereview.chromium.org",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "native_client@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@  \"patch_failure\": false, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"patch_root\": \"native_client\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision\": \"a95eb4c044a154d692a53fdfa564ee8de7aeea5b\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision_cp\": \"refs/heads/master@{#256160}\"@@@",
@@ skipping 44 matching lines @@
       "--rietveld_url",
       "https://codereview.chromium.org",
       "--rietveld_fetch",
       "--upstream",
       "a95eb4c044a154d692a53fdfa564ee8de7aeea5b",
       "--trybot-json",
       "/path/to/tmp/json",
       "--rietveld_email",
       ""
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "presubmit",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@[@@@",
       "@@@STEP_LOG_LINE@json.output@  [@@@",
       "@@@STEP_LOG_LINE@json.output@    \"chromium_presubmit\", @@@",
       "@@@STEP_LOG_LINE@json.output@    [@@@",
       "@@@STEP_LOG_LINE@json.output@      \"compile\"@@@",
       "@@@STEP_LOG_LINE@json.output@    ]@@@",
       "@@@STEP_LOG_LINE@json.output@  ]@@@",
       "@@@STEP_LOG_LINE@json.output@]@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   }
 ]