Recipes: make cwd default to slave_build

scripts/slave/recipes/mojo.expected/mojo_win_dbg.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.testing.master",
       "--builder",
       "Mojo Win (dbg)",
       "--slave",
       "TestSlavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/external/mojo.git'}]",
       "--root",
       "src",
       "--revision_mapping_file",
       "{}",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@HEAD",
       "--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\": \"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@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/mojo/tools/mojob.py",
       "gn",
       "--debug"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "mojob gn"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/mojo/tools/mojob.py",
       "build",
       "--debug"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "mojob build"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/mojo/tools/get_test_list.py",
       "{\"build_number\": 571, \"builder_name\": \"Mojo Win (dbg)\", \"dcheck_always_on\": false, \"is_debug\": true, \"master_name\": \"chromium.testing.master\", \"target_os\": \"windows\", \"test_results_server\": \"test-results.appspot.com\", \"test_types\": [\"default\"]}",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "get_test_list",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@[@@@",
       "@@@STEP_LOG_LINE@json.output@  {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"command\": [@@@",
       "@@@STEP_LOG_LINE@json.output@      \"world\"@@@",
       "@@@STEP_LOG_LINE@json.output@    ], @@@",
       "@@@STEP_LOG_LINE@json.output@    \"name\": \"Hello\"@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
       "@@@STEP_LOG_LINE@json.output@]@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "world"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "Hello"
   }
 ]