Add grunt clean to postinstall temporarily.

Author: desrosj

package.json

@@ -112,7 +112,7 @@
 		"wicg-inert": "3.1.3"
 	},
 	"scripts": {
-		"postinstall": "npm run gutenberg:verify",
+		"postinstall": "npm run grunt clean -- --dev && npm run gutenberg:verify",
 		"build": "grunt build",
 		"build:dev": "grunt build --dev",
 		"build:gutenberg": "grunt build:gutenberg",