Build: Fix Gutenberg build base-url argument passing across platforms.

When building Gutenberg, the `--base-url` argument was forwarded through
`npm run build --`, which passes arguments through a shell layer. The
argument value `includes_url( 'build/' )` contains spaces, parentheses,
and single quotes that could be mangled by shell parsing on some
platforms, causing the generated `constants.php` to lose the trailing
slash in `build_url`.

This invokes `node bin/build.mjs` directly instead, bypassing npm's
shell forwarding entirely. The argument is passed as a single array
element via `spawn`, so it arrives intact regardless of platform.

Props youknowriad, desrosj.
See #64656.

Built from https://develop.svn.wordpress.org/trunk@61679


git-svn-id: http://core.svn.wordpress.org/trunk@60987 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: youknowriad

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.0-alpha-61678';
+$wp_version = '7.0-alpha-61679';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.