guix system: Make 'init' idempotent again.

This fixes a regression introduced in
df2f6400: since the new
'register-path' (actually 'reset-timestamps') would make files
read-only, 'delete-file-recursively' would fail to delete them.  Thus,
re-running 'guix system init' on an already-populated store would fail
with a 'delete-file' EPERM.

* guix/scripts/system.scm (copy-item): Use 'lstat' instead of
'file-exists?'.  Call 'make-file-writable' on each directory below
DEST.

guix/scripts/system.scm

@@ -126,7 +126,11 @@ (define* (copy-item item references target
     ;; Remove DEST if it exists to make sure that (1) we do not fail badly
     ;; while trying to overwrite it (see <http://bugs.gnu.org/20722>), and
     ;; (2) we end up with the right contents.
-    (when (file-exists? dest)
+    (when (false-if-exception (lstat dest))
+      (for-each make-file-writable
+                (find-files dest (lambda (file stat)
+                                   (eq? 'directory (stat:type stat)))
+                            #:directories? #t))
       (delete-file-recursively dest))
 
     (copy-recursively item dest