arrow-cpp: fix RPATH on darwin

Instead of patching test binaries with install_name_tool just set
CMAKE_SKIP_BUILD_RPATH to OFF let cmake compile them with RPATH set
correctly.

Add CMAKE_INSTALL_RPATH to set correct RPATH for the installed
executables for parquet tools. We could set it to an absolute path using
"-DCMAKE_INSTALL_RPATH=${placeholder "out"}", but instead we use
@loader_path to provide a relocable solution.