Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

scripts/slave/recipes/chromium_trybot.expected/full_tryserver_blink_mac_blink_rel.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.blink",
       "--builder",
       "mac_blink_rel",
@@ skipping 311 matching lines @@
       "tryserver.blink",
       "-b",
       "mac_blink_rel",
       "--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 (with patch)"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/clang_revision.py",
       "--src-dir",
       "[SLAVE_BUILD]/src",
@@ skipping 126 matching lines @@
       "@@@STEP_LINK@layout_test_results@https://storage.googleapis.com/chromium-layout-test-archives/mac_blink_rel/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/mac_blink_rel/571/layout-test-results.zip@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]