Build/Test Tools: Force run grunt clean --dev before build.

Seemingly as a side effect of [61438], the built code base contains a number of files and directories that have been moved or removed.

`grunt clean` was updated to be aware of the newly expected files in [62051], but the `clean` tasks required to ensure a pristine working state are not run prior to `build`.

This runs the `clean --dev` task is run immediately before `grunt build` runs as an attempt to clean up these files by ensuring a clean `src` directory.

Props jorbin.
See #64393.

git-svn-id: https://develop.svn.wordpress.org/trunk@62069 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

Gruntfile.js

@@ -2063,9 +2063,19 @@ module.exports = function(grunt) {
 	] );
 
 	grunt.registerTask( 'build', function() {
+		var done = this.async();
+		
+		grunt.util.spawn( {
+			grunt: true,
+			args: [ 'clean', '--dev' ],
+			opts: { stdio: 'inherit' }
+		}, function( buildError ) {
+			done( ! buildError );
+		} );
+
 		if ( grunt.option( 'dev' ) ) {
 			grunt.task.run( [
-				'gutenberg:verify',
+				'gutenberg:download',
 				'build:js',
 				'build:css',
 				'build:codemirror',
@@ -2075,7 +2085,7 @@ module.exports = function(grunt) {
 			] );
 		} else {
 			grunt.task.run( [
-				'gutenberg:verify',
+				'gutenberg:download',
 				'build:certificates',
 				'build:files',
 				'build:js',