Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_chromium_rel_ng",
@@ skipping 284 matching lines @@
       "-m",
       "tryserver.chromium.linux",
       "-b",
       "linux_chromium_rel_ng",
       "-v",
       "//out/Release",
       "{\"additional_compile_targets\": [\"chromium_swarm_tests\"], \"files\": [\"foo.cc\"], \"test_targets\": []}",
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "analyze",
     "~followup_annotations": [
       "@@@STEP_TEXT@No compile necessary@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"compile_targets\": [], @@@",
       "@@@STEP_LOG_LINE@json.output@  \"status\": \"No dependency\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"test_targets\": []@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LOG_LINE@analyze_details@{@@@",
@@ skipping 16 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "mark: before_tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]