Recipes: make cwd default to slave_build

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "tryserver.libyuv",
       "--builder",
       "ios_rel",
       "--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'}]\ntarget_os = ['ios', 'mac']",
       "--root",
       "src",
       "--revision_mapping_file",
       "{}",
       "--patch_url",
       "try_job_svn_patch",
       "--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_CROSSCOMPILE": "1",
       "GYP_DEFINES": "OS=ios clang=1 component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[BUILD]/goma' target_arch=arm use_goma=1",
       "GYP_GENERATORS": "ninja"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/compile.py",
       "--target",
       "Release-iphoneos",
       "--src-dir",
       "[SLAVE_BUILD]/src",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma-clang",
       "--goma-dir",
       "[BUILD]/goma",
       "--",
       "All"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "compile"
   }
 ]