CI is broken on `master`

The CI currently fails to build, giivng with this error message on the update-repo stage. Here is an excerpt from the log at https://gitlab.haskell.org/ghc/head.hackage/-/jobs/133431:

$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable nixpkgs
$ nix-channel --update
unpacking channels...
$ nix build -f scripts/build-repo.nix
builder for '/nix/store/j4vrimrjpzmxpcz34y80k7gkvzmb0wdd-source.drv' failed with exit code 1; last 7 log lines:
  
  trying https://github.com/haskell-vanguard/hackage-security/archive/5ce34d42ffa9d760dafcf216a10d6f72bd82a1d3.tar.gz
    % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                   Dload  Upload   Total   Spent    Left  Speed
    0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  curl: (22) The requested URL returned error: 404 Not Found
  error: cannot download source from any mirror
cannot build derivation '/nix/store/wh28wh1xbi6mq3ldhg2yycw3gb7zzkv5-cabal2nix-hackage-repo-tool.drv': 1 dependencies couldn't be built
error: build of '/nix/store/wh28wh1xbi6mq3ldhg2yycw3gb7zzkv5-cabal2nix-hackage-repo-tool.drv' failed
(use '--show-trace' to show detailed location information)

Running after script...
$ rm -Rf keys

ERROR: Job failed: exit code 1

The URL https://github.com/haskell-vanguard/hackage-security does not exist, so I'm not sure why it's trying to download something from there.