The cache path we get from the `llvm::sys::path::cache_directory()`
library function is not writeable on some Windows bots as they point us
to the Default user's AppData: C:\Users\Default\AppData\Local\lldb.
This patch avoids the problem and downloads into the temporary directory
that we query from `llvm::sys::path::system_temp_directory()`. Once we
introduce real caching, this would have changed anyway.
621f03240b