THRIFT-5887: ensure using our CMake modules

Client: cpp

If we append our CMake module path to `CMAKE_MODULE_PATH` and Apache
Thrift is built with FetchContent, our CMake modules in `build/cmake/`
may not be used. Because other paths in `CMAKE_MODULE_PATH` may be
used. For example, both of Apache Arrow and Apache Thrift has
`DefineOptions.cmake`. If Apache Arrow builds Apache Thrift with
FetchContent, Apache Arrow's `DefineOptions.cmake` is used
unexpectedly.