Recipes: make cwd default to slave_build

scripts/slave/recipes/cros/cbuildbot.expected/chromiumos_paladin.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[gitiles]/resources/gerrit_client.py",
       "--json-file",
       "/path/to/tmp/json",
       "--url",
       "https://chromium.googlesource.com/chromiumos/manifest-versions/+/a94a8fe5ccb19ba61c4c0873d391e987982fbbd3?format=json",
       "--attempts",
       "10"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "Fetch manifest config",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br/>Build ID: 1337<br/>Chromite branch: master@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"author\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"email\": \"testauthor@fake.chromium.org\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"name\": \"Test Author\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"time\": \"Mon Jan 01 00:00:00 2015\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"commit\": \"a94a8fe5ccb19ba61c4c0873d391e987982fbbd3\", @@@",
@@ skipping 28 matching lines @@
       "--root",
       "chromite",
       "--revision_mapping_file",
       "{}",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "chromite@master:",
       "--force"
     ],
+    "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\": \"chromite\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {}, @@@",
       "@@@STEP_LOG_LINE@json.output@  \"root\": \"chromite\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"step_text\": \"Some step text\"@@@",
@@ skipping 12 matching lines @@
       "master",
       "--buildnumber",
       "12345",
       "--master-build-id",
       "1337",
       "x86-generic-paladin"
     ],
     "name": "cbuildbot [x86-generic-paladin]"
   }
 ]