* Prioritize DVC_SITE_CACHE_DIR over core.site_cache_dir
* Add tests for proper prioritization of env var and config
* Tests require python-3.9 compatible typing, use Optional
* Remove comment
* Fixup
* DVC_SITE_CACHE_DIR must be sent first, deal with platformdirs afterwards
* Test should check default behaviour when no env var is set
* Add test: site_cache_dir() must return the env var if set