packages: Build tarballs in sorted order even if tar doesn't support it.

This is a followup to commit 92226a47.

* guix/packages.scm (patch-and-repack)[build]: Determine if tar supports the
  "--sort=name" option using a run-time test.  If not supported, generate the
  sorted file list with 'find-files' and pass it to tar using "--files-from".