I just built what you built and got

error: hash mismatch in fixed-output derivation '/nix/store/qkk0kml8q992k99ivj83bdf7fri7j8d7-discord-0.6.1-go-modules.drv':
         specified: sha256-GKUy/HYuPyaUNzfAD7NQWH+tuwcoQhWU4bfxhOpIM0Y=
            got:    sha256-8SfxzooeX9jPdi2FZ4iiGiF8e3DLtZhjOvFnNsvPAjU=

I don't think you invalidated your vendorsha256 when you changed something so it's using an old copy of the modules that's out of sync

FYI this builds for me using latest nixpkgs repo and

$ nix --version
nix (Nix) 2.15.1

{ lib
, buildGoModule
, fetchFromGitHub
, olm
}:

buildGoModule rec {
  pname = "discord";
  version = "0.6.1";

  src = fetchFromGitHub {
    owner = "mautrix";
    repo = "discord";
    rev = "v${version}";
    hash = "sha256-rs7wWlQMc79Vls+cqPPo+lRzYAGye4WcKKz+9EXlEBo=";
  };

  vendorSha256 = "sha256-ZI1+Tfru2OfnqLnaaiDL08OtSmbMBiRDvkL39+jhhmI=";

  ldflags = [ "-s" "-w" ];

  buildInputs = [ olm ];

  doCheck = false;

  meta = with lib; {
    description = "A Matrix-Discord puppeting bridge";
    homepage = "https://github.com/mautrix/discord";
    changelog = "https://github.com/mautrix/discord/blob/${src.rev}/CHANGELOG.md";
    license = licenses.agpl3Only;
    maintainers = with maintainers; [ ];
  };
}

• system: "x86_64-linux"
  
  • host os: Linux 6.4.11, NixOS, 23.11 (Tapir), 23.11.20230823.9201b5f
  • multi-user?: no
  • sandbox: yes
  • version: nix-env (Nix Super) 2.18.0pre20230813_65e8aba
  • channels(root): ""
  • channels(moritz): ""
  • nixpkgs: not found

• system: "x86_64-linux"
• host os: Linux 6.4.11, NixOS, 23.11 (Tapir), 23.11.20230823.9201b5f
• multi-user?: no
• sandbox: yes
• version: nix-env (Nix Super) 2.18.0pre20230813_65e8aba
• channels(root): ""
• channels(moritz): ""
• nixpkgs: not found

hmm ok so I thought it might be sandboxing disabled but it's on.
I'm not sure how you managed to turn off multi-user on nixos ... 👀
nix 2.18 probably didn't break anything.

is this file in a clone of nixpkgs? how are you running nix-build?

wrote a tiny idiomatic API wrapper around C bindings to nix:

\$ go run examples/build\_hello/main.go
drv /nix/store/nms6chyaxb6pxzb021jx351nn32j3lx7-hello-2.12.1.drv vaild? true
out -> /nix/store/6wmplzfs0nz866p320xnknpr05q79m62-hello-2.12.1

wrote a tiny idiomatic API wrapper around C bindings to nix:

$ go run examples/build_hello/main.go
drv /nix/store/nms6chyaxb6pxzb021jx351nn32j3lx7-hello-2.12.1.drv vaild? true
out -> /nix/store/6wmplzfs0nz866p320xnknpr05q79m62-hello-2.12.1