[cmake] [swig] use CMake's built-in file-copying mechanisms instead of 'cp'

[jameslamb]

As described in #5972, the use of COMMAND cp in this project's CMake code leads to errors when the project is built in environments where cp is not available (for example, the Windows CMD shell by default).

This proposes replacing all of those uses with COMMAND cmake -e copy_if_different, so that cp isn't strictly required to build LightGBM.

[jameslamb]

Replacing "*.dll" with 2 specific files here because cmake -E copy_if_different does not support wildcards.

• https://gitlab.kitware.com/cmake/cmake/-/issues/16791
• Add note about cmake -E copy not supporting wildcards Kitware/CMake#301
• https://cmake.org/cmake/help/latest/manual/cmake.1.html#cmdoption-cmake-E-arg-copy

[jameslamb]

Thanks for all the reviews @guolinke !