Restore `rapidsai` channel before the `26.10` release

[csadorf]

#221 temporarily removed the stable rapidsai channel from release/26.10 to avoid incompatible RAPIDS XGBoost packages. This must be reverted before the release because published environments cannot rely on rapidsai-nightly.

• Restore rapidsai after rapidsai-nightly.
• Confirm release environments resolve without selecting RAPIDS XGBoost packages.

Blocked by rapidsai/build-planning#312.

Follow-up to #221 (comment)

Related: #222 and NVIDIA/cuml#8602