Respect $ZSH_CACHE_DIR when creating fasd's cache

plugins/fasd/fasd.plugin.zsh

 if [ $commands[fasd] ]; then # check if fasd is installed
-  fasd_cache="$HOME/.fasd-init-cache"
+  fasd_cache="${ZSH_CACHE_DIR}/fasd-init-cache"
   if [ "$(command -v fasd)" -nt "$fasd_cache" -o ! -s "$fasd_cache" ]; then
     fasd --init auto >| "$fasd_cache"
   fi