Login and Registration: Send `nocache_headers()` on Multisite signup pages.

Props herregroen.
Fixes #43843.

git-svn-id: https://develop.svn.wordpress.org/trunk@43030 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-signup.php

@@ -7,6 +7,8 @@ add_action( 'wp_head', 'wp_no_robots' );
 
 require( dirname( __FILE__ ) . '/wp-blog-header.php' );
 
+nocache_headers();
+
 if ( is_array( get_site_option( 'illegal_names' ) ) && isset( $_GET['new'] ) && in_array( $_GET['new'], get_site_option( 'illegal_names' ) ) ) {
 	wp_redirect( network_home_url() );
 	die();