chromium.run_mb: set cwd to checkout

scripts/slave/recipes/chromium_trybot.expected/swarming_trigger_failure.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]/linux_chromium_rel_ng",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 377 matching lines @@
       "-b",
       "linux_chromium_rel_ng",
       "--config-file",
       "[BUILDER_CACHE]/linux_chromium_rel_ng/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--swarming-targets-file",
       "base_unittests\n",
       "//out/Release"
     ],
+    "cwd": "[BUILDER_CACHE]/linux_chromium_rel_ng/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files (with patch)",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@base_unittests@@@",
       "@@@STEP_LOG_END@swarming-targets-file.txt@@@"
     ]
   },
   {
@@ skipping 143 matching lines @@
       "@@@SET_BUILD_PROPERTY@failure_hash@\"212d0eb2385b1eaa828053689225258ece065725\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 out of 1 aggregated steps failed. Failures: Step('base_unittests') failed with return_code 1",
     "recipe_result": null,
     "status_code": 1
   }
 ]