Correct how Xdebug is loaded

plugins/templates/before-script.yml

@@ -16,5 +16,5 @@ before_script:
   - composer global require yoast/phpunit-polyfills
 
   # Enable Xdebug for coverage reports
-  - echo "extension=xdebug.so" > /usr/local/etc/php/conf.d/xdebug.ini
+  - echo "zend_extension=xdebug.so" > /usr/local/etc/php/conf.d/xdebug.ini
   - echo "xdebug.mode=coverage" >> /usr/local/etc/php/conf.d/xdebug.ini