Meta Boxes: Use a unique name for the nonce of the meta box loader.

Fixes a case where saving in the block editor fails if there are two `_wpnonce` arguments in the request, one overriding the other so that `use_block_editor_for_post()` wasn't able to check the nonce properly.

Props Chouby.
See #45253.

git-svn-id: https://develop.svn.wordpress.org/trunk@44938 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/edit-form-blocks.php

@@ -117,10 +117,10 @@ wp_add_inline_script(
 $meta_box_url = admin_url( 'post.php' );
 $meta_box_url = add_query_arg(
 	array(
-		'post'            => $post->ID,
-		'action'          => 'edit',
-		'meta-box-loader' => true,
-		'_wpnonce'        => wp_create_nonce( 'meta-box-loader' ),
+		'post'                  => $post->ID,
+		'action'                => 'edit',
+		'meta-box-loader'       => true,
+		'meta-box-loader-nonce' => wp_create_nonce( 'meta-box-loader' ),
 	),
 	$meta_box_url
 );