Nix-darwin rebuild fails when inside a git repository

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Nix noob here.

I have nix-darwin installed and created a flake.nix file:

flake.nix 

  description = "Example Darwin system flake";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-24.05-darwin";
    nix-darwin = {
      url = "github:LnL7/nix-darwin";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    home-manager = {
      url = "github:nix-community/home-manager/release-24.05";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs = inputs@{ self, nix-darwin, nixpkgs, home-manager }:
  let
    configuration = { pkgs, ... }: {
      # List packages installed in system profile. To search by name, run:
      # $ nix-env -qaP | grep wget
      environment.systemPackages = [
          pkgs.vim
        ];
      system.stateVersion = 4;

      # The platform the configuration will be used on.
      nixpkgs.hostPlatform = "x86_64-darwin";
    };
  in
  {
    # Build darwin flake using:
    # $ darwin-rebuild build --flake .#xx
    darwinConfigurations = {
      "xx" = nix-darwin.lib.darwinSystem {
        modules = [
          configuration
          home-manager.darwinModules.home-manager {
            home-manager.useGlobalPkgs = true;
            home-manager.useUserPackages = true;
          }
        ];
      };
    };

    # Expose the package set, including overlays, for convenience.
    darwinPackages = self.darwinConfigurations."xx".pkgs;
  };

I have put that in a directory ~/x and ran the following which gives no errors

$ darwin-rebuild switch --dry-run --flake x/
building the system configuration...
Password:
user defaults...
setting up user launchd services...
setting up /Applications/Nix Apps...
setting up pam...
applying patches...
setting up /etc...
system defaults...
setting up launchd services...
reloading nix-daemon...
waiting for nix-daemon
configuring networking...
setting up /Library/Fonts/Nix Fonts...
setting nvram variables...

However, if add the directory to git, it gives an error which gives me no clue how to fix

$ cp -r x y
$ cd y
$ git init && git add . && git commit -am 'initial'

$ darwin-rebuild switch --dry-run --flake y/
building the system configuration...
these 9 derivations will be built:
  /nix/store/6q8gs4030fj613h1rc77qdd2vmsiydn7-options.json.drv
  /nix/store/6i4fwbq03hrbsvw9skl60zz8w82z9mzp-options.json.drv
  /nix/store/1213z7hj01zs50wsy3yjqsj6lcdvw4p7-darwin-manpages.drv
  /nix/store/xdby0n64ijkcmyl1wf9dby5fgvmin03a-darwin-manual-html.drv
  /nix/store/7m19k5gppfcbisdzk0rlyhqcq2kqsq88-darwin-help.drv
  /nix/store/b35vswyff9pfd2a57y85zc8r326fsdq9-system-path.drv
  /nix/store/ss6wmn08c1zp3snng97hs1qb7pl7x9my-system-applications.drv
  /nix/store/z5c666bd7d5lrhg879n46hyl7mvp7a0j-darwin-version.json.drv
  /nix/store/b1wjlf0rnzj84mv3w1yffqa1rph62jnc-darwin-system-24.05.20240621.201ed88+darwin4.29b3096.drv
don't know how to build these paths:
  /nix/store/a2j86yi5rbprnq9wis88pq5n4qyfm0gq-darwin-system-24.05.20240621.201ed88+darwin4.29b3096
error: build of '/nix/store/a2j86yi5rbprnq9wis88pq5n4qyfm0gq-darwin-system-24.05.20240621.201ed88+darwin4.29b3096' failed

What am I doing wrong? Keen to know what’s happening here not just how to fix it.

Versions 
$ darwin-version
24.05.20240619.dd457de+darwin4.29b3096

$ nix --version
nix (Nix) 2.18.2