Global Header: Bring back custom property for header offset

This value is used elsewhere in the Local Navigation Bar CSS, and in the parent theme. Removing it causes alignment issues when the value is undefined.

Follow-up to #665
Fixes #672

mu-plugins/blocks/global-header-footer/postcss/_common.pcss

@@ -21,6 +21,8 @@
 html {
 	--wp--custom--alignment--scroll-bar-width: 8px;
 
+	/* Offset from the top of the page. */
+	--wp-global-header-offset: calc(var(--wp-global-header-height, 0px) + var(--wp-admin--admin-bar--height, 0px));
 	height: unset; /* Let height use default browser height. */
 }