Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_chromium_rel_ng",
@@ skipping 290 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_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"invalid_targets\": [@@@",
       "@@@STEP_LOG_LINE@json.output@    \"invalid target\", @@@",
       "@@@STEP_LOG_LINE@json.output@    \"another one\"@@@",
       "@@@STEP_LOG_LINE@json.output@  ]@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "import sys; sys.exit(0)"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "mark: before_tests",
     "~followup_annotations": [
       "@@@SET_BUILD_PROPERTY@failure_hash@\"3908b365fae78e6b96f5e3b38db5b33131cde606\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "Error, following targets were not found: invalid target, another one",
     "status_code": 1
   }
 ]