Correct two issues found when testing CMake 3.27 rc2

[robertmaynard]

Description

Corrects two small logic errors that became exposed due to changes in ExternalProject in CMake 3.27 which made it less robust to invalid inputs.

The rapids_cpm_thrust function now properly supports the DOWNLOAD_ONLY parameter, and rapids_cpm_rmm now propagates arguments correctly to the underlying CPMFindPackage.

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.

[robertmaynard]

/merge