nixos/image: add image.repart.package option

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.

nixos/modules/image/repart.nix

@@ -88,6 +88,13 @@ in
       '';
     };
 
+    package = lib.mkPackageOption pkgs "systemd-repart" {
+      default = "systemd";
+      example = lib.literalExpression ''
+        pkgs.systemdMinimal.override { withCryptsetup = true; }
+      '';
+    };
+
     partitions = lib.mkOption {
       type = with lib.types; attrsOf (submodule partitionOptions);
       default = { };
@@ -178,9 +185,9 @@ in
       in
       pkgs.runCommand cfg.name
         {
-          nativeBuildInputs = with pkgs; [
-            fakeroot
-            systemd
+          nativeBuildInputs = [
+            cfg.package
+            pkgs.fakeroot
           ] ++ fileSystemTools;
         } ''
         amendedRepartDefinitions=$(${amendRepartDefinitions} ${partitions} ${definitionsDirectory})