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;
...
};
};
```
Klemens Nanni