Recipes: make cwd default to slave_build

scripts/slave/recipes/dart/dartium_build.expected/basic_win_64_MirrorFalse.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]\\gclient.py",
       "config",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'dartium.deps', 'url': 'https://dart.googlecode.com/svn/branches/bleeding_edge/deps/dartium.deps'}]"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "gclient setup"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]\\gclient.py",
       "sync",
       "--nohooks",
       "--force",
       "--verbose",
       "--delete_unversioned_trees",
       "--revision",
       "dartium.deps@12345",
       "--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@    \"dartium.deps/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": 10382@@@",
       "@@@STEP_LOG_LINE@json.output@    }, @@@",
       "@@@STEP_LOG_LINE@json.output@    \"src/dart/\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"revision\": 58622@@@",
       "@@@STEP_LOG_LINE@json.output@    }, @@@",
@@ skipping 23 matching lines @@
       "@@@SET_BUILD_PROPERTY@got_v8_revision@43426@@@"
     ]
   },
   {
     "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' linux_strip_binary=1 target_arch=x64 use_goma=1",
       "GYP_GENERATORS": "ninja"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\compile.py",
       "--target",
       "Release_x64",
       "--src-dir",
       "[SLAVE_BUILD]\\src",
       "--build-tool",
       "ninja",
       "--compiler",
       "goma",
       "--goma-dir",
       "[BUILD]\\goma",
       "--",
       "All",
       "chromium_builder_tests"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "compile"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]\\src\\dart\\tools\\dartium\\multivm_archive.py",
       "12345"
     ],
+    "cwd": "[SLAVE_BUILD]",
     "name": "archive_build"
   }
 ]