Extending @prosti answer, WordPress add it using action login_head
in wp-login.php
itself.
add_action( 'login_head', 'wp_no_robots' );
You can remove this action in theme/plugin and add your own action with custom callback function.
Example:-
//Keep priority 9 so we can remove WordPress action that is on 10
add_action( 'login_head', 'custom_no_robots', 9);
/**
* Custom robot tags
*/
function custom_no_robots() {
remove_action( 'login_head', 'wp_no_robots' );
echo "<meta name="robots" content="noindex, nofollow" />\n";
}