Don't show <nix/derivation.nix> in stack traces

Messages like

  while evaluating the attribute `outPath' at `/nix/store/212ngf4ph63mp6p1np2bapkfikpakfv7-nix-1.6/share/nix/corepkgs/derivation.nix:18:9':

are redundant, because Nix already shows that it's evaluating a derivation:

  while instantiating the derivation named `firefox-24.0' at `/home/eelco/Dev/nixpkgs/pkgs/applications/networking/browsers/firefox/default.nix:131:5':
  while evaluating the derivation attribute `nativeBuildInputs' at `/home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/default.nix:76:17':

src/libexpr/eval.cc

@@ -639,7 +639,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
         state.forceValue(*vAttrs);
 
     } catch (Error & e) {
-        if (pos)
+        if (pos && pos->file != state.sDerivationNix)
             addErrorPrefix(e, "while evaluating the attribute `%1%' at %2%:\n",
                 showAttrPath(attrPath), *pos);
         throw;

src/libexpr/eval.hh

@@ -95,6 +95,7 @@ public:
 
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls;
+    Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
        already exist there. */

src/libexpr/primops.cc

@@ -1262,7 +1262,9 @@ void EvalState::createBaseEnv()
 
     /* Add a wrapper around the derivation primop that computes the
        `drvPath' and `outPath' attributes lazily. */
-    evalFile(findFile("nix/derivation.nix"), v);
+    string path = findFile("nix/derivation.nix");
+    sDerivationNix = symbols.create(path);
+    evalFile(path, v);
     addConstant("derivation", v);
 
     /* Now that we've added all primops, sort the `builtins' attribute