daemon: Define 'NIX_STORE' before invoking 'guix perform-download'.

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'.

nix/libstore/builtins.cc

@@ -21,6 +21,7 @@
 #include <globals.hh>
 
 #include <unistd.h>
+#include <cstdlib>
 
 namespace nix {
 
@@ -41,6 +42,11 @@ static void builtinDownload(const Derivation &drv,
 	"download", drvPath.c_str(), output.c_str(), NULL
       };
 
+    /* Tell the script what the store file name is, so that
+       'strip-store-file-name' (used for instance to determine the URL of
+       content-addressed mirrors) works correctly.  */
+    setenv("NIX_STORE", settings.nixStore.c_str(), 1);
+
     /* XXX: Hack our way to use the 'download' script from 'LIBEXECDIR/guix'
        or just 'LIBEXECDIR', depending on whether we're running uninstalled or
        not.  */