gn error on mac release build

[simonhong]

ERROR at //chrome/BUILD.gn:753:63: Undefined identifier in string expansion.
        sources += [ "app/theme/$branding_path_component/mac/$brave_channel/app.icns", ]
                                                              ^------------
"brave_channel" is not currently in scope.
See //BUILD.gn:100:7: which caused the file to be included.
      "//chrome",
      ^---------
null
null
error An unexpected error occurred: "Command failed.
Exit code: 1
Command: sh
Arguments: -c node ./scripts/commands.js build Release