fix: find zstd libraries on Mac for 0.12.x (#172)

Plugins~/Build/cmake_modules/ZSTD.cmake

@@ -20,7 +20,7 @@ function(setup_zstd meshsync_plugin_path)
         set(zstd_lib_path_suffix "win64")
     elseif(APPLE)
         set(zstd_lib_filename "libzstd.a")
-        set(zstd_lib_path_suffix "osx")
+        set(zstd_lib_path_suffix "osx/${CMAKE_SYSTEM_PROCESSOR}/")
     elseif(LINUX)
         set(zstd_lib_filename "libzstd.a")
         set(zstd_lib_path_suffix "linux64")