It tells you the problem, install-my-fonts isn’t a proper package either. You should use runCommand not writeScriptBin.
Also you can’t access the network in non-FOD packages, use a fetcher.
https://nix.dev/tutorials/packaging-existing-software
https://nixos.org/manual/nixpkgs/stable/#chap-pkgs-fetchers
https://nixos.org/manual/nixpkgs/stable/#trivial-builder-runCommand
Also, I just said above to remove this line: