Recipes: make cwd default to slave_build

scripts/slave/recipes/skia/skia.expected/Build-Ubuntu-GCC-x86_64-Debug.json

 [
   {
     "cmd": [
       "git",
       "fetch"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
     "name": "git fetch"
   },
   {
@@ skipping 18 matching lines @@
   },
   {
     "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'}]"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient setup"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "sync",
       "--nohooks",
       "--force",
       "--verbose",
       "--delete_unversioned_trees",
       "--revision",
       "skia@abc123",
       "--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": [
       "python",
       "-u",
       "\nimport os\nimport sys\nwith open(sys.argv[1], 'w') as f:\n  f.write(os.path.join(os.path.expanduser('~'), 'src'))\n",
       "/path/to/tmp/"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "get CHROME_PATH",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import os@@@",
       "@@@STEP_LOG_LINE@python.inline@import sys@@@",
       "@@@STEP_LOG_LINE@python.inline@with open(sys.argv[1], 'w') as f:@@@",
       "@@@STEP_LOG_LINE@python.inline@  f.write(os.path.join(os.path.expanduser('~'), 'src'))@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "cmd": [
       "make",
       "most",
       "BUILDTYPE=Debug"
     ],
     "cwd": "[SLAVE_BUILD]/skia",
     "env": {
       "CHROME_PATH": "/home/chrome-bot/src",
       "GYP_DEFINES": "skia_arch_type=x86_64 skia_arch_width=64 skia_warnings_as_errors=1"
     },
     "name": "build most"
   }
 ]