guix system: Fix bootloader config file generation by 'guix system roll-back'.

Fixes <https://bugs.gnu.org/33623>.
Reported by Diego Nicola Barbato <dnbarbato@posteo.de>.

Regression introduced in commit
46c296dc.

* guix/scripts/system.scm (reinstall-bootloader): Add call to
'lower-object'.

guix/scripts/system.scm

@@ -497,9 +497,10 @@ (define (reinstall-bootloader store number)
          (old-entries (map boot-parameters->menu-entry old-params)))
     (run-with-store store
       (mlet* %store-monad
-          ((bootcfg ((bootloader-configuration-file-generator bootloader)
-                     bootloader-config entries
-                     #:old-entries old-entries))
+          ((bootcfg (lower-object
+                     ((bootloader-configuration-file-generator bootloader)
+                      bootloader-config entries
+                      #:old-entries old-entries)))
            (bootcfg-file -> (bootloader-configuration-file bootloader))
            (target -> "/")
            (drvs -> (list bootcfg)))