Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

scripts/slave/recipes/chromium.expected/full_chromium_chromiumos_ChromiumOS_x86_generic_Compile.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.chromiumos",
       "--builder",
       "ChromiumOS x86-generic Compile",
@@ skipping 237 matching lines @@
       "chromium.chromiumos",
       "-b",
       "ChromiumOS x86-generic Compile",
       "--config-file",
       "[SLAVE_BUILD]/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "//out/Release"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "generate_build_files"
   },
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[depot_tools]/cros",
       "chrome-sdk",
       "--board=x86-generic",
       "--nocolor",
       "--use-external-config",
       "--gomadir",
@@ skipping 67 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "package build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]