admin: Use linkcopy_sync_data() for initramfs instead of rename()

It's possible (likely even) that /tmp is on a separate filesystem; in
that case, a raw rename() is going to fail.

Saw this on the ostree.gnome.org builder.

src/ostree/ot-admin-builtin-update-kernel.c

@@ -175,7 +175,8 @@ generate_initramfs (OtAdminUpdateKernel  *self,
           goto out;
         }
 
-      if (!gs_file_rename (initramfs_tmp_file, ret_initramfs_path,
+      if (!gs_file_linkcopy_sync_data (initramfs_tmp_file, ret_initramfs_path,
+                                       G_FILE_COPY_OVERWRITE,
                                        cancellable, error))
         goto out;