Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

scripts/slave/recipes/cronet.expected/android_cronet_builder__dbg_.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys\n\nbuild_path = sys.argv[1]\nif os.path.exists(build_path):\n  for (path, dir, files) in os.walk(build_path):\n    for cur_file in files:\n      if cur_file.endswith('index.lock'):\n        path_to_file = os.path.join(path, cur_file)\n        print 'deleting %s' % path_to_file\n        os.remove(path_to_file)\n",
       "[SLAVE_BUILD]"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "cleanup index.lock",
@@ skipping 165 matching lines @@
       "chromium.testing.master",
       "-b",
       "Android Cronet Builder (dbg)",
       "--config-file",
       "[SLAVE_BUILD]/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[BUILD]/goma",
       "//out/Debug"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "generate_build_files"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/compile.py",
@@ skipping 423 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@  os.remove(report)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]