Set WP Admin > Settings > Reading robots access //* Discourage search engines from indexing this site

functions.php

//* Discourage search engines from indexing this site
add_action('init', 'gumtheme_set_robots_access');
function gumtheme_set_robots_access(){
    if(WP_ENV == 'staging' && get_option('blog_public') == '1') 
    	update_option('blog_public', '0');
    
    if(WP_ENV == 'production' && get_option('blog_public') == '0') 
    	update_option('blog_public', '1');
}