Fixes#2506
``XDG_CACHE_HOME`` does not work on Windows. To allow for users to set a custom cache directory on all systems I added a new environment variable ``BLACK_CACHE_DIR`` to set the cache directory. The default remains the same so users will only notice a change if that environment variable is set.
The specific use case I have for this is I need to run black on in different processes at the same time. There is a race condition with the cache pickle file that made this rather difficult. A custom cache directory will remove the race condition.
I created ``get_cache_dir`` function in order to test the logic. This is only used to set the ``CACHE_DIR`` constant.
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