luxrays: rename GetConfigDir as GetCacheDir and use XDG_CACHE_HOME according to specifications

[illwieckz]

Rename GetConfigDir as GetCacheDir and use XDG_CACHE_HOME according to specifications.

• ~/.config should only be used as a fallback if XDG_CONFIG_HOME isn't set.
• ~/.config or XDG_CONFIG_HOME is a config directory, not a cache directory.
• the cache directory is XDG_CACHE_HOME, is not set by default and should fallback on ~/.cache.
• getenv returns nullptr but boost::filesystem::path doesn't accept nullptr.

This work is courtesy of I ♥ Compute initiative funded by 🇫🇷️ rebatir.fr.