nar: Really really protect the temporary store directory from GC.

nar: Really really protect the temporary store directory from GC.

This is a follow-up to 6071b55e10b7b6e67d77ae058c8744834889e0b4.
See <https://lists.gnu.org/archive/html/guix-devel/2014-04/msg00167.html>
for the original report, and
<https://lists.gnu.org/archive/html/guix-devel/2014-04/msg00198.html>
for an alternate solution that has been discussed.

* guix/nar.scm (temporary-store-file): Remove call to
  'add-permanent-root'; don't loop.
  (with-temporary-store-file): Rewrite using 'with-store' and
  'add-temp-root'.