Recipes: make cwd default to slave_build

scripts/slave/recipes/libyuv/libyuv.expected/client_libyuv_win32_release__vs2012_.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\bot_update.py",
       "--master",
       "client.libyuv",
       "--builder",
       "Win32 Release (VS2012)",
       "--slave",
       "slavename",
       "--spec",
       "cache_dir = None\nsolutions = [{'custom_vars': {'root_dir': 'src'}, 'deps_file': 'DEPS', 'managed': True, 'name': 'src', 'url': 'https://chromium.googlesource.com/external/libyuv'}]",
       "--root",
       "src",
       "--revision_mapping_file",
       "{}",
       "--output_json",
       "/path/to/tmp/json",
       "--revision",
       "src@12345",
       "--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]",
     "env": {
       "GYP_DEFINES": "chromium_win_pch=0 component=static_library fastbuild=1 gomadir='[BUILD]\\goma' target_arch=ia32 use_goma=1",
       "GYP_MSVS_VERSION": "2012"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\compile.py",
       "--target",
       "Release",
       "--src-dir",
       "[SLAVE_BUILD]\\src",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma",
       "--goma-dir",
       "[BUILD]\\goma",
       "--",
       "All"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "compile"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"Win32 Release (VS2012)\", \"mastername\": \"client.libyuv\", \"recipe\": \"libyuv/libyuv\", \"revision\": \"12345\", \"slavename\": \"slavename\"}",
       "--build-properties",
       "{\"BUILD_CONFIG\": \"Release\", \"buildername\": \"Win32 Release (VS2012)\", \"mastername\": \"client.libyuv\", \"recipe\": \"libyuv/libyuv\", \"revision\": \"12345\", \"slavename\": \"slavename\"}",
       "--builder-name=Win32 Release (VS2012)",
       "--slave-name=slavename",
       "libyuv_unittest.exe"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "libyuv_unittest"
   }
 ]