Build: Move Gutenberg checkout to a postinstall script.

This separates dependency setup from the build process by running
the Gutenberg checkout during npm install rather than before each
build. Build times decrease by approximately 30 seconds since the
checkout now occurs once during installation.

Props youknowriad, dmsnell.
See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@61458 602fd350-edb4-49c9-b593-d223f7449a82
(cherry picked from commit bc567fd)

Author: youknowriad

Gruntfile.js

@@ -1457,7 +1457,6 @@ module.exports = function(grunt) {
 	} );
 
 	grunt.registerTask( 'gutenberg-integrate', 'Complete Gutenberg integration workflow.', [
-		'gutenberg-checkout',
 		'gutenberg-build',
 		'gutenberg-copy'
 	] );

package-lock.json

@@ -97,6 +97,7 @@
 		"wicg-inert": "3.1.3"
 	},
 	"scripts": {
+		"postinstall": "npm run gutenberg:checkout",
 		"build": "grunt build",
 		"build:dev": "grunt build --dev",
 		"dev": "grunt watch --dev",