Recently, a Haskell build job has started failing: https://travis-ci.org/github/sebastiaanvisser/clay/jobs/693190286
The crucial part seems to be:
$ nix-env --version
nix-env (Nix) 2.0.4
$ nix-instantiate --eval -E 'with import <nixpkgs> {}; lib.version or lib.nixpkgsVersion'
"20.09pre227892.b27a19d5bf7"
0.98s$ nix-build
error: unknown hash algorithm '', at /nix/store/fhksmnm8dk9b1hs99sfzha4nwvpfs86n-nixpkgs-20.09pre227892.b27a19d5bf7/nixpkgs/pkgs/build-support/fetchurl/default.nix:119:3
(use '--show-trace' to show detailed location information)
The command "nix-build" exited with 1.
When I try to reproduce locally, everything is fine:
$ nix-env --version
nix-env (Nix) 2.3.3
$ nix-build
these derivations will be built:
[...]
This succeeds without errors.
Note that it seems that my Nix version is newer.
cc @domenkozar @garbas