Use require to retrieve the app's config

This uses loader.js's `require` function to retrieve the app's environment config. This bypasses the `importSync` dependency checking that happens in Embroider builds which causes the build to fail since the dependency doesn't exist in the addon.

addon/index.js

@@ -1,8 +1,6 @@
-import { getOwnConfig, importSync } from '@embroider/macros';
+/* global require */
+import { getOwnConfig } from '@embroider/macros';
 
 let configModulePath = `${getOwnConfig().modulePrefix}/config/environment`;
 
-let config = importSync(configModulePath);
-
-// fix problem with fastboot config being wrapped in a second "default" object
-export default config.default?.default ?? config.default;
+export default require(configModulePath).default;