nixUnstable.perlBindings: Fix Nix::Config generation

The configure script calls nix-instantiate, which fails if /nix/var
doesn't exist (e.g. in a sandbox). This caused a bogus Nix::Config
module to be generated, causing issues in Hydra.

pkgs/tools/package-management/nix/default.nix

@@ -116,9 +116,10 @@ let
     configureFlags =
       [ "--with-dbi=${perlPackages.DBI}/${perl.libPrefix}"
         "--with-dbd-sqlite=${perlPackages.DBDSQLite}/${perl.libPrefix}"
-        "--with-www-curl=${perlPackages.WWWCurl}/${perl.libPrefix}"
       ];
 
+    preConfigure = "export NIX_STATE_DIR=$TMPDIR";
+
     preBuild = "unset NIX_INDENT_MAKE";
   };