Weirdly, enough, golang is one of the only programming languages that actually has built-in support for a cross-OS config dir location: [os.UserConfigDir()][1].

I don't really ever program in golang, but whenever I write a Node.JS/Python tool that does need a user-global config file, I just write my own implementation of it:

  function userConfigDir() {
    switch (process.platform) {
      case 'darwin':
        return `${os.homedir()}/Library/Application Support`;
      case 'win32':
        if (process.env['APPDATA']) {
          return process.env['APPDATA'];
        } else {
          throw new Error('%APPDATA% is not set correctly');
        }
      case 'aix':
      case 'freebsd':
      case 'openbsd':
      case 'sunos':
      case 'linux':
        return process.env['XDG_CONFIG_HOME'] || `${os.homedir()}/.config`;
      default:
        throw new Error(`The platform ${process.platform} is currently unsupported.`);
    }
  }

[1]: https://pkg.go.dev/os#UserConfigDir