Follow XDG directory specifications

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

`${SORRI_CACHE_DIR_PREFIX:-$HOME/.cache/sorri/${SORRI_CACHE_NAME}}"` does not follow the spec for users may want their caches. Use `$XDG_CACHE_DIR` as the highest priority and fallback to `$HOME` after.