vm: 'expression->derivation-in-linux-vm' leads to a kernel panic upon failure.

Partially fixes <https://bugs.gnu.org/34276>.
Reported by Tobias Geerinckx-Rice <me@tobias.gr>.

* gnu/system/vm.scm (expression->derivation-in-linux-vm)[loader]: Call
'exit' when USER-BUILDER exits with a non-zero code.

gnu/system/vm.scm

@@ -187,8 +187,9 @@ made available under the /xchg CIFS share."
                   ;; When USER-BUILDER succeeds, reboot (indicating a
                   ;; success), otherwise die, which causes a kernel panic
                   ;; ("Attempted to kill init!").
-                  #~(when (zero? (system* #$user-builder))
-                      (reboot))))
+                  #~(if (zero? (system* #$user-builder))
+                        (reboot)
+                        (exit 1))))
 
   (let ((initrd (or initrd
                     (base-initrd file-systems