Allow setting custom cache directory on all platforms (#2739)

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.