Write client config to filename.

def GetClientConfig(filename):
  """Write client config to filename."""
  config_lib.SetPlatformArchContext()
  config_lib.ParseConfigCommandLine()
  context = list(grr_config.CONFIG.context)
  context.append("Client Context")
  deployer = build.ClientRepacker()
  # Disable timestamping so we can get a reproducible and cachable config file.
  config_data = deployer.GetClientConfig(
      context, validate=True, deploy_timestamp=False)
  builder = build.ClientBuilder()
  with open(filename, "w") as fd:
    fd.write(config_data)
    builder.WriteBuildYaml(fd, build_timestamp=False)