Recipes: make cwd default to slave_build

scripts/slave/recipes/skia/skia.expected/has_ccache_android.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "config",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'skia', 'url': 'https://skia.googlesource.com/skia.git'}]\ntarget_os = ['android']"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient setup"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "sync",
       "--nohooks",
       "--force",
       "--verbose",
       "--delete_unversioned_trees",
       "--output-json",
       "/path/to/tmp/json"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient sync",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"solutions\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"skia/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": 164710@@@",
       "@@@STEP_LOG_LINE@json.output@    }@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@got_revision@164710@@@"
     ]
   },
   {
     "cmd": [
       "which",
       "ccache"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "has ccache?",
     "stdout": "/path/to/tmp/"
   },
   {
     "cmd": [
       "[SLAVE_BUILD]/skia/platform_tools/android/bin/android_ninja",
       "most",
       "-d",
       "arm_v7_neon"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
     "env": {
       "ANDROID_MAKE_CCACHE": "/usr/bin/ccache",
       "ANDROID_SDK_ROOT": "/home/chrome-bot/android-sdk-linux",
       "BUILDTYPE": "Debug",
       "GYP_DEFINES": "skia_arch_type=arm skia_arch_width=32 skia_warnings_as_errors=1",
       "SKIA_ANDROID_VERBOSE_SETUP": "1"
     },
     "name": "build most"
   }
 ]