Update scripts.md (#11880)

Updates the 'Setting environment variables' example to call Composer using `@composer` in order to automatically resolve to whatever composer.phar is currently being used as detailed in the previous section on this same page.

doc/articles/scripts.md

@@ -419,7 +419,7 @@ To set an environment variable in a cross-platform way, you can use `@putenv`:
     "scripts": {
         "install-phpstan": [
             "@putenv COMPOSER=phpstan-composer.json",
-            "composer install --prefer-dist"
+            "@composer install --prefer-dist"
         ]
     }
 }