Reported by rohit yadav <rohityadav@utexas.edu>
at <https://lists.gnu.org/archive/html/guix-devel/2017-02/msg00191.html>.
This fixes a regression whereby 'guix perform-download' would always see
NIX_STORE as unset and thus use "/gnu/store", leading it to miscompute
the hydra.gnu.org content-addressed URLs when the store file name is not
"/gnu/store".
* nix/libstore/builtins.cc (builtinDownload): Add 'setenv' call for
'NIX_STORE'.