Use the new RAPIDS.cmake to fetch rapids-cmake (#838)

The original approach of using FetchContent naively has a subtle bug when multiple projects that use rapids-cmake are combined together inside as sibling projects. This bug causes any `include(rapids-*)` commands to fail, causing CMake errors.

But using `RAPIDS.cmake` we can resolve this issue and remove the new complex logic from each consumer.

Authors:
  - Robert Maynard (https://github.com/robertmaynard)

Approvers:
  - Mark Harris (https://github.com/harrism)

URL: #838

CMakeLists.txt

@@ -14,12 +14,10 @@
 
 cmake_minimum_required(VERSION 3.20.1 FATAL_ERROR)
 
-include(FetchContent)
-FetchContent_Declare(
-  rapids-cmake
-  GIT_REPOSITORY https://github.com/rapidsai/rapids-cmake.git
-  GIT_TAG origin/branch-21.08)
-FetchContent_MakeAvailable(rapids-cmake)
+file(DOWNLOAD https://raw.githubusercontent.com/rapidsai/rapids-cmake/branch-21.10/RAPIDS.cmake
+     ${CMAKE_BINARY_DIR}/RAPIDS.cmake)
+include(${CMAKE_BINARY_DIR}/RAPIDS.cmake)
+
 include(rapids-cmake)
 include(rapids-cpm)
 include(rapids-export)