add_filter('robots_txt', 'wpse_248124_robots_txt', 10,  2);

function wpse_248124_robots_txt($output, $public) {

  return 'YOUR DESIRED OUTPUT';
}

# Don't log access to /robots.txt
location = /robots.txt {
    access_log    off;
    log_not_found off;
}

# Don't log access to /robots.txt
location = /robots.txt {
    try_files     $uri $uri/ /index.php?$args;
    access_log    off;
    log_not_found off;
}