Prevent Project Config (warnings) in production

[ostark]

The actual changes happen in fortrabbit/craft-auto-migrate. This additional check assures that the composer plugin is not blocked.

Context: https://getcomposer.org/doc/06-config.md#allow-plugins

Sister tickets

• PR https://github.com/fortrabbit/craft-auto-migrate/pull/10
• Issue https://github.com/craftcms/cms/issues/11971