Recipes: make cwd default to slave_build

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "chromium_presubmit",
@@ skipping 13 matching lines @@
       "https://codereview.chromium.org",
       "--apply_issue_email_file",
       "[BUILD]/site_config/.rietveld_client_email",
       "--apply_issue_key_file",
       "[BUILD]/site_config/.rietveld_secret_key",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "infra_internal@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\": \"infra_internal\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"properties\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision\": \"0a18b32e3a93913e1090d1ee6b8965c6dc3116d6\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"got_revision_cp\": \"refs/heads/master@{#190894}\"@@@",
@@ skipping 46 matching lines @@
       "--rietveld_fetch",
       "--upstream",
       "0a18b32e3a93913e1090d1ee6b8965c6dc3116d6",
       "--trybot-json",
       "/path/to/tmp/json",
       "--rietveld_email_file",
       "[BUILD]/site_config/.rietveld_client_email",
       "--rietveld_private_key_file",
       "[BUILD]/site_config/.rietveld_secret_key"
     ],
+    "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@@@"
     ]
   }
 ]