fcitx5-rime: ensure 'default.yaml' exists to prevent startup issues
Some Rime schemas, such as `rime-wanxiang`, avoid conflicts with `rime-data` by renaming
their own `default.yaml` to a different name (e.g., `*_suggestion.yaml`) during packaging.
This enables users to selectively import them via patches.
However, if a user installs only this package without `rime-data`, Rime fails to start
because `default.yaml` is missing.
To prevent this issue, we ensure `default.yaml` is present in `postBuild` of `symlinkJoin`.