Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

scripts/slave/recipes/findit/chromium/test.expected/test_without_targets_not_skipped.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "taskkill"
   },
@@ skipping 547 matching lines @@
       "-m",
       "chromium.win",
       "-b",
       "Win Builder",
       "-v",
       "//out/Release",
       "{\"additional_compile_targets\": [], \"files\": [\"foo.cc\"], \"test_targets\": [\"unaffected_tests\", \"unaffected_tests_run\"]}",
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
+    },
     "name": "test r1.analyze",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@",
       "@@@STEP_TEXT@No compile necessary@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"compile_targets\": [], @@@",
       "@@@STEP_LOG_LINE@json.output@  \"status\": \"No dependencies\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"test_targets\": []@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
@@ skipping 80 matching lines @@
       "@@@STEP_LOG_END@report@@@",
       "@@@SET_BUILD_PROPERTY@report@{\"metadata\": {\"regression_range_size\": 1}, \"result\": {\"r1\": {\"checkperms\": {\"status\": \"passed\", \"valid\": true}, \"unaffected_tests\": {\"status\": \"skipped\", \"valid\": true}}}}@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]