Function default_cache_dir 

pub fn default_cache_dir() -> NormalizedPath

Returns the configured cache directory path.

If ZCCACHE_CACHE_DIR is set and non-empty, it is used as the cache root. Relative override paths are made absolute against the current working directory so the daemon and CLI derive the same subpaths when spawned together. If unset, this falls back to ~/.zccache on all platforms.