Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

scripts/slave/recipes/findit/chromium/compile.expected/compile_skipped.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_variable",
@@ skipping 502 matching lines @@
       "-m",
       "chromium.linux",
       "-b",
       "Linux Builder",
       "-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": "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 35 matching lines @@
       "@@@STEP_LOG_END@report@@@",
       "@@@SET_BUILD_PROPERTY@report@{\"metadata\": {\"regression_range_size\": 1, \"sub_ranges\": [[null, \"r1\"]]}, \"result\": {\"r1\": \"skipped\"}}@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]