CI/Build: Build Gutenberg before installing Core dependencies.

Because of a strange interaction with building Gutenberg from within the plugins directory, TypeScript was appearing to examine code for which it should have no access up the filesystem hierarchy.

This patch changes the order of building Gutenberg so that it occurs before those parent directories contain TypeScript code (by installing the `npm` dependencies) and thus frees up the builds.

This change should not serve as a workaround to avoid fixing the root issue, introduced in 8401993599. Rather, it should be seen as at least a temporary way to remove the trigger of the problem during CI builds.

See also alternative work:

 - To build Gutenberg outside of the plugin directory in https://github.com/WordPress/wordpress-develop/pull/10282/files.
 - To patch the broken `@wordpress/blocks` types inside of `@wordpress/core-data`.

Developed in https://github.com/WordPress/wordpress-develop/pull/10283

Props czarate, desrosj, dmsnell, ellatrix, jorbin, peterwilsoncc, ramonopoly, westonruter, youknowriad.

Built from https://develop.svn.wordpress.org/trunk@60985


git-svn-id: http://core.svn.wordpress.org/trunk@60321 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.9-alpha-60984';
+$wp_version = '6.9-alpha-60985';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.