Only apply install rules for CCCL if we actually downloaded

[bdice]

Description

This fixes rapids_cpm_cccl to only call the logic for install rules if CCCL was downloaded.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The cmake-format.json is up to date with these changes.
• I have added new files under rapids-cmake/
  • I have added include guards (include_guard(GLOBAL))
  • I have added the associated docs/ rst file and update the api.rst

[bdice]

/merge