Cosmic-idle: developer shell broken

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1

I have the following flake:

flake.nix

{
  inputs = {
    stable.url = "github:nixos/nixpkgs/nixos-25.11";
    # ...
  };

  outputs = inputs@{stable, ...}: {
    # ...
    nixosConfigurations.my-system = stable.lib.nixosSystem {
      modules = [
        ({...}: {
          config = {
            nix.registry.stable.flake = stable;
            # ...
          };
        })
      ];
    };
  };
}

If I do nix develop stable#cosmic-idle then I’m getting the following output:

bash: export: `CARGO_TARGET_@cargoLinkerVar@_RUSTFLAGS=': not a valid identifier

any ideas why I’m getting this?