nixos/systemd: Package only built component units

Account for all `with*` options causing their respective unit files to
not be built, just like the current code `withCryptsetup` already does.

This fixes build errors like the following:
```
missing /nix/store/5fafsfms64fn3ywv274ky7arhm9yq2if-systemd-250.4/example/systemd/system/systemd-importd.service
error: builder for '/nix/store/67rdli5q5akzwmqgf8q0a1yp76jgr0px-system-units.drv' failed with exit code 1
```

Found by using a customised systemd package as follows:
```
systemd.package = pkgs.systemd-small;

nixpkgs.config.packageOverrides = pkgs: {

  "systemd-small" = pkgs.systemd.override {
    withImportd = false;
    withMachined = false;
    ...
  };

};
```

0 files changed, 0 insertions, 0 deletions