Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.webkit",
       "--builder",
       "WebKit Mac Builder",
@@ skipping 271 matching lines @@
       "WebKit Mac Builder",
       "--config-file",
       "[SLAVE_BUILD]/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--swarming-targets-file",
       "browser_tests\n",
       "//out/Release"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "generate_build_files",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@browser_tests@@@",
       "@@@STEP_LOG_END@swarming-targets-file.txt@@@"
     ]
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
@@ skipping 175 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@WebKit Mac10.11@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]