Modify post-root-package-install script.

Idempotence.

composer.json

@@ -30,7 +30,7 @@
     },
     "scripts": {
         "post-root-package-install": [
-            "php -r \"copy('.env.example', '.env');\""
+            "php -r \"file_exists('.env') || copy('.env.example', '.env');\""
         ],
         "post-create-project-cmd": [
             "php artisan key:generate"