Where to download a VM image with installed `x86_64-freebsd`?

... because the derivations require a `x86_64-freebsd` builder and it is not available to download. I know, there suppose to be a binary cache, but it does not list its files to download, so a user has to try all commit history in hope that some commit will produce a valid hash to download a VM image from the binary cache (e.g. the last commit does not)