Fix GOMA_SERVICE_ACCOUNT_JSON_FILE propagation in mb.

scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Win_x64_GN.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "taskkill"
   },
@@ skipping 264 matching lines @@
       "chromium.webrtc.fyi",
       "-b",
       "Win x64 GN",
       "--config-file",
       "[SLAVE_BUILD]\\src\\tools\\mb\\mb_config.pyl",
       "--goma-dir",
       "[CACHE]\\cipd\\goma",
       "//out/Release_x64"
     ],
     "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "C:\\creds\\service_accounts\\service-account-goma-client.json"
+    },
     "name": "generate_build_files"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\tools\\runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\compile.py",
@@ skipping 56 matching lines @@
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "package build"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]