Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

scripts/slave/recipes/chromium.expected/full_chromium_webkit_WebKit_Win_x64_Builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "taskkill"
   },
@@ skipping 270 matching lines @@
       "WebKit Win x64 Builder",
       "--config-file",
       "[SLAVE_BUILD]\\src\\tools\\mb\\mb_config.pyl",
       "--goma-dir",
       "[CACHE]\\cipd\\goma",
       "--swarming-targets-file",
       "browser_tests\n",
       "//out/Release_x64"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\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@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 139 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "process_dumps"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]