guix system: Fix 'init' again.

Fixes a regression introduced in
52ee4479, whereby 'install' would now be
passed a procedure instead of a lowerable object.

* guix/scripts/system.scm (perform-action): Pass BOOTLOADER-SCRIPT as
the #:bootloader-installer argument of 'install'.

guix/scripts/system.scm

@@ -910,7 +910,7 @@ (define bootloader-script
                       #:install-bootloader? install-bootloader?
                       #:bootcfg bootcfg
                       #:bootcfg-file bootcfg-file
-                      #:bootloader-installer bootloader-installer))
+                      #:bootloader-installer bootloader-script))
             (else
              ;; All we had to do was to build SYS and maybe register an
              ;; indirect GC root.