Consider %LocalAppData% for Windows

[dimo414]

Currently bkt defaults to using the directory returned by std::env::temp_dir() on all platforms. In principle bkt should probably use a "user cache" directory instead, such as ProjectDirs::cache_dir - notably this would isolate cached data between users, which is currently a limitation we workaround on Unix with permissions.

However, in practice using temp_dir() is probably the best choice on Linux/OSX because we want to use a tempfs directory for performance. On Windows (to my knowledge) that isn't likely/possible so we might as well use %LocalAppData% instead which shouldn't be any slower than the Windows temp directory.