Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_chromium_rel_ng",
@@ skipping 358 matching lines @@
       "linux_chromium_rel_ng",
       "--config-file",
       "[SLAVE_BUILD]/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--swarming-targets-file",
       "gl_tests\n",
       "//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)",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@gl_tests@@@",
       "@@@STEP_LOG_END@swarming-targets-file.txt@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 353 matching lines @@
       "linux_chromium_rel_ng",
       "--config-file",
       "[SLAVE_BUILD]/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--swarming-targets-file",
       "gl_tests\n",
       "//out/Release"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
     "name": "generate_build_files (without patch)",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@gl_tests@@@",
       "@@@STEP_LOG_END@swarming-targets-file.txt@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 230 matching lines @@
       "@@@SET_BUILD_PROPERTY@failure_type@\"TEST_FAILURE\"@@@",
       "@@@SET_BUILD_PROPERTY@failure_hash@\"2c817ab2d4394c69c9b8a0698b954936d9e1fb76\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 out of 1 aggregated steps failed. Failures: Step('gl_tests (retry summary)') failed with return_code 1",
     "status_code": 1
   }
 ]