Allow giving a custom package containing the `systemd-repart` binary.
Defaults to `pkgs.systemd`. This option opens up the possibility to use
a different package for the image builder and the system configuration.
For example, someone could use this option to build an image with a
patched systemd while still using the upstream nixpkgs systemd package
(i.e., `pkgs.systemd`) for the system configuration installed to the
created image.
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