chromium.run_mb: set cwd to checkout

scripts/slave/recipes/chromium.expected/full_chromium_chromiumos_ChromiumOS_x86_generic_Compile.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[BUILDER_CACHE]/ChromiumOS_x86_generic_Compile",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 260 matching lines @@
       "-m",
       "chromium.chromiumos",
       "-b",
       "ChromiumOS x86-generic Compile",
       "--config-file",
       "[BUILDER_CACHE]/ChromiumOS_x86_generic_Compile/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "//out_x86-generic/Release"
     ],
+    "cwd": "[BUILDER_CACHE]/ChromiumOS_x86_generic_Compile/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files"
   },
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[depot_tools]/cros",
       "chrome-sdk",
       "--board=x86-generic",
@@ skipping 74 matching lines @@
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]/build_data_dir\", \"buildername\": \"ChromiumOS x86-generic Compile\", \"buildnumber\": 571, \"mastername\": \"chromium.chromiumos\", \"parent_buildername\": null, \"path_config\": \"kitchen\", \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]