Avoiding passthru itself was the right direction, but not a
complete solution. Passthru attributes typically do not contain
dependencies, but rather extra paths that are not relevant to
the build itself.
Whether we want to include all (passthru) test dependencies this
way can be debated, but removing them makes this expression more
robust.
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.
