chromium.run_mb: set cwd to checkout

scripts/slave/recipes/chromium.expected/full_chromium_fyi_ClangToTMac__dbg_.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]/ClangToTMac__dbg_",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 200 matching lines @@
       "[BUILDER_CACHE]/ClangToTMac__dbg_/src/tools/mb/mb.py",
       "gen",
       "-m",
       "chromium.fyi",
       "-b",
       "ClangToTMac (dbg)",
       "--config-file",
       "[BUILDER_CACHE]/ClangToTMac__dbg_/src/tools/mb/mb_config.pyl",
       "//out/Debug"
     ],
+    "cwd": "[BUILDER_CACHE]/ClangToTMac__dbg_/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
@@ skipping 111 matching lines @@
       "LLVM_FORCE_HEAD_REVISION": "YES"
     },
     "name": "sizes"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]