Motivation for this change

In contrast to .service-units, it's not possible to declare an
overrides.conf, however this is done by generateUnits for .nspawn
units as well. This change breaks the build if you have two derivations
configuring one nspawn unit.

This will happen in a case like this:

{ pkgs, ... }: {
  systemd.packages = [
    (pkgs.writeTextDir "etc/systemd/nspawn/container0.nspawn" ''
      [Files]
      Bind=/tmp
    '')
  ];
  systemd.nspawn.container0 = {
    /* ... */
  };
}

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.