image: operating-system-for-image: Bring back root-fs placeholder.

To calculate the UUID in `root-uuid`, the operating-system has to have a root
file system, because it's typically used in the
operating-system-boot-parameters.

Followup of efc32c6684 that it partially reverts.

* gnu/system/image.scm (operating-system-for-image): Re-introduce root file
system to operating-system used to calculate the UUID.

Fixes: #5131
Reported-By: Yelninei <yelninei@tutamail.com>
Change-Id: Ica2c3d3715cd90fccf3a7484f390b36de3cdfe7f
Signed-off-by: Rutherther <rutherther@ditigal.xyz>
Merges: #5141

gnu/system/image.scm

@@ -1034,7 +1034,12 @@ it can be used for bootloading."
                               (inherit
                                (operating-system-bootloader base-os))
                               (bootloader grub-mkrescue-bootloader))
-                            (operating-system-bootloader base-os)))))
+                            (operating-system-bootloader base-os)))
+            (file-systems (cons (file-system
+                                  (mount-point "/")
+                                  (device "/dev/placeholder")
+                                  (type root-file-system-type))
+                                file-systems-to-keep))))
          (uuid (root-uuid os)))
          (operating-system
            (inherit os)