Fix missing assets in iOS IPA workflow

[Copilot]

The iOS IPA GitHub Actions job failed during PPC source generation because default.xex and other private build assets were unavailable.

• Workflow changes
  • Check out the configured private asset repository.
  • Copy its contents into UnleashedRecompLib/private before host-side generation.

- name: Checkout private assets
  uses: actions/checkout@v4
  with:
    repository: ${{ secrets.ASSET_REPO }}
    token: ${{ secrets.ASSET_REPO_TOKEN }}
    path: ./private

- name: Prepare project assets
  run: cp ./private/* ./UnleashedRecompLib/private