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'.