Linux: Store cache in the ~/.cache/czkawka directory

[alexislefebvre]

Since #139 (which was a great addition!), the hashes of similar images are stored in a cache.

On Linux, the cache is stored in ~/.config/czkawka: https://github.com/qarmin/czkawka/blob/9a0432d237b21f6a7951bc6d6a9f06c5a3047bd1/instructions/Instruction.md#configcache-files

I think it would be better to store the cache in ~/.cache/czkawka. The freedesktop spec recommend to use it:

$XDG_CACHE_HOME defines the base directory relative to which user specific non-essential data files should be stored. If $XDG_CACHE_HOME is either not set or empty, a default equal to $HOME/.cache should be used.

Source: https://specifications.freedesktop.org/basedir-spec/latest/ar01s03.html