Adds `noindex` to the robots meta tag for embeds.

Typical usage is as a {@see ‘wp_robots’} callback:

add_filter( ‘wp_robots’, ‘wp_robots_noindex_embeds’ );

function wp_robots_noindex_embeds( array $robots ) {
	if ( is_embed() ) {
		return wp_robots_no_robots( $robots );
	}

	return $robots;
}