Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_chromium_rel_ng",
@@ skipping 323 matching lines @@
       "tryserver.chromium.linux",
       "-b",
       "linux_chromium_rel_ng",
       "--config-file",
       "[SLAVE_BUILD]/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "//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)"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/compile.py",
@@ skipping 151 matching lines @@
       "tryserver.chromium.linux",
       "-b",
       "linux_chromium_rel_ng",
       "--config-file",
       "[SLAVE_BUILD]/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "//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)"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/compile.py",
@@ skipping 45 matching lines @@
     "~followup_annotations": [
       "@@@SET_BUILD_PROPERTY@failure_hash@\"4d6a903860330c798a14ca4fb8a3b9ea8c90b814\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "Step('compile (with patch)') failed with return_code 1",
     "status_code": 1
   }
 ]