Okay, here's a way to reproduce:

{
  inputs.cask.url = "https://formulae.brew.sh/api/cask.jws.json";
  inputs.cask.flake = false;

  outputs = {self, nixpkgs, cask}: {
    packages = nixpkgs.lib.genAttrs nixpkgs.lib.systems.flakeExposed (system: {
      test = nixpkgs.legacyPackages.${system}.runCommand "test" {} ''
        cp ${cask} $out
      '';
    });
  };
}

Do nix build .#test, wait a while for the URL to get updated (you can check the Last-Modified/Expires headers and https://github.com/Homebrew/formulae.brew.sh/actions, might be good to give it some extra time for things to propagate), then:

emily@yuyuko ~/D/url-weirdness-test> nix flake archive
error: NAR hash mismatch in input 'https://formulae.brew.sh/api/cask.jws.json?narHash=sha256-UBfAYZ9000fvnJCC4xXMWWpeSDbexDTqoG97RlHXgGc=' (/nix/store/1r9vipfw5gp2imjbwkxzhszrciw6lbvc-source), expected 'sha256-UBfAYZ9000fvnJCC4xXMWWpeSDbexDTqoG97RlHXgGc=', got 'sha256-MgLlVZ5GaSiFlFZbbtRW3dYqA786GtdtKBIiOntj5AY='

i'd like to add a neovim plugin for use in a devShell for a specific project. below is my current flake. i'm using flake-parts, and the general sentiment is that flakes should not be relying on overlays. i've read on a discourse thread that i should "build on top on nixpkgs" instead. i have no idea what that phrase means, or what it would look like. any tips?

{
  description = "Basic dotnet7 dev environment";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    flake-parts.url = "github:hercules-ci/flake-parts";
    flake-root.url = "github:srid/flake-root";
    mission-control.url = "github:Platonic-Systems/mission-control";
  };

  outputs = inputs@{ self, nixpkgs, flake-parts, ...}:
    flake-parts.lib.mkFlake { inherit inputs; } {
      systems = nixpkgs.lib.systems.flakeExposed;
      imports = [
        inputs.flake-root.flakeModule
        inputs.mission-control.flakeModule
      ];
      perSystem = { pkgs, lib, config, ... }:
        let
          omnisharp = pkgs.omnisharp-roslyn;

          nvimrc = ''...'';
        in
        {
          # Executed by 'nix build'
            #empty...
          # Used by `nix develop`
          mission-control.scripts = {
            dnbuild = {
              description = "dotnet build";
              exec = "boxxy -d -l warn dotnet build";
            };
          };
          devShells.default = pkgs.mkShell {
            inputsFrom = [ config.mission-control.devShell ];
            buildInputs = with pkgs; [
              # Sandboxing
              boxxy
              #basics
              dotnet-sdk_7
              dotnet-runtime

              #packages
              dotnetPackages.Nuget #install nuget declaratively

              #unit testing
              dotnetPackages.NUnit
              dotnetPackages.NUnitRunners

              #language server of choice
              omnisharp-roslyn
            ];

            DOTNET_CLI_TELEMETRY_OPTOUT = 1;
            DOTNET_SKIP_FIRST_TIME_EXPERIENCE = "true";

            shellHook = ''
              echo '${nvimrc}' > .nvimrc.lua
            '';
          };
        };
      };
}