I managed to solve this by first setting packNupkg = false in the flake file then regenerating dependencies like:

rm ./deps.nix
touch ./deps.nix
nix build .#default.passthru.fetch-deps
./result deps.nix

Afterwards, nix build worked fine

Here is what the definition looks like now, for reference:

      packages = forAllSystems (system:
        let
          pkgs = nixpkgsFor.${system};
        in {
          default = pkgs.buildDotnetModule rec {
            pname = "inventory-backend";
            inherit version;

            src = ./backend;
            projectFile = "backend.sln";

            # To generate deps.nix:
            #
            # nix develop
            # dotnet restore backend/ --packages out
            # nuget-to-nix out > deps.nix
            # OR
            # nix build .#default.passthru.fetch-deps
            nugetDeps = ./deps.nix;

            dotnet-sdk = pkgs.dotnet-sdk_8;
            dotnet-runtime = pkgs.dotnet-aspnetcore_8;

            packNupkg = false;
          };
        });