Find-tarballs was failing on some `requireFile` that was fixed-output
but didn't specify `outputHashMode`. Apparently, nix is fine with that
and uses the "flat" default, although that's undocumented.
Tarballs.nixos.org is now stored in an S3 bucket rather than an EBS
volume. Redirects are used to simulate symlinks.
The function find-tarballs.nix now filters out fetchzip, fetchpatch
and the like.