Use file(COPY ) over file(INSTALL ) so cmake output is reduced (#7616)

Currently it can be hard to see important details in cudf CMake output. The biggest culprit is the usage of `file(INSTALL` which outputs per file the current status on each configuration, as shown below:
```
- Up-to-date: /home/nfs/rmaynard/Work/cudf/cpp/build/cuda-11.0/branch-0.19/release/include/libcxx/include/<file.h>
```
Moving to `file(COPY` has all the same behavior but doesn't have any output.

Authors:
  - Robert Maynard (@robertmaynard)

Approvers:
  - Keith Kraus (@kkraus14)

URL: #7616

cpp/cmake/Modules/StringifyJITHeaders.cmake

@@ -164,5 +164,5 @@ add_custom_target(stringify_run DEPENDS
 # - copy libcu++ ----------------------------------------------------------------------------------
 
 # `${LIBCUDACXX_INCLUDE_DIR}/` specifies that the contents of this directory will be installed (not the directory itself)
-file(INSTALL "${LIBCUDACXX_INCLUDE_DIR}/" DESTINATION "${CUDF_GENERATED_INCLUDE_DIR}/include/libcudacxx")
-file(INSTALL "${LIBCXX_INCLUDE_DIR}"      DESTINATION "${CUDF_GENERATED_INCLUDE_DIR}/include/libcxx")
+file(COPY "${LIBCUDACXX_INCLUDE_DIR}/" DESTINATION "${CUDF_GENERATED_INCLUDE_DIR}/include/libcudacxx")
+file(COPY "${LIBCXX_INCLUDE_DIR}"      DESTINATION "${CUDF_GENERATED_INCLUDE_DIR}/include/libcxx")