make-derivation: fix position in trace

For a long time now, tracing has been broken in Nixpkgs. So when you
have an eval error you would get something like this:

  error: while evaluating the attribute 'buildInputs' of the derivation 'hello-2.10' at /home/mbauer/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:185:11:
  while evaluating 'chooseDevOutputs' at /home/mbauer/nixpkgs/lib/attrsets.nix:474:22, called from undefined position:
  while evaluating 'optionals' at /home/mbauer/nixpkgs/lib/lists.nix:257:5, called from /home/mbauer/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:132:17:

This is coming from how Nix handles string context and how
make-derivation messes with the "name" attribute. This commit should
restore the old behavior so you get a nice line number like:

  error: while evaluating the attribute 'buildInputs' of the derivation 'hello-2.10' at /home/mbauer/nixpkgs/pkgs/applications/misc/hello/default.nix:4:3:
  while evaluating 'chooseDevOutputs' at /home/mbauer/nixpkgs/lib/attrsets.nix:474:22, called from undefined position:
  while evaluating 'optionals' at /home/mbauer/nixpkgs/lib/lists.nix:257:5, called from /home/mbauer/nixpkgs/pkgs/stdenv/generic/make-derivation.nix:132:17:

NOTE: This will still be broken for cross compilation due to the
prefixes we are adding to name.