primops/storePath: add trace to pure mode error

As described in #4745 it's otherwise fairly hard to understand where
this is coming from. Say you have an expression which uses e.g.
`types.package`:

``` nix
{ outputs = { self, nixpkgs }: {
    packages.x86_64-linux.hello = let
      foo = nixpkgs.lib.evalModules {
        modules = [
          {
            options.foo.bar = with nixpkgs.lib; mkOption { type = types.package; };
          }
          {
            foo.bar = ./.;
          }
        ];
      };
    in builtins.trace foo.config.foo.bar.outPath nixpkgs.legacyPackages.x86_64-linux.hello;
    defaultPackage.x86_64-linux = self.packages.x86_64-linux.hello;
  };
}
```

Then you'll get an error trace like this:

```
error: 'builtins.storePath' is not allowed in pure evaluation mode

       at /nix/store/p4h2x6r80njkb0j2rc1xjhhl99yri3zb-source/lib/attrsets.nix:328:15:

          327|     let
          328|       path' = builtins.storePath path;
             |               ^
          329|       res =

       … while evaluating the attribute 'config.foo.bar.outPath'

       at /nix/store/p4h2x6r80njkb0j2rc1xjhhl99yri3zb-source/lib/attrsets.nix:332:11:

          331|           name = sanitizeDerivationName (builtins.substring 33 (-1) (baseNameOf path'));
          332|           outPath = path';
             |           ^
          333|           outputs = [ "out" ];

       … while evaluating the attribute 'packages.x86_64-linux.hello'

       at /nix/store/6c1rfsqzrhjw1235palzjmf5vihcpci7-source/flake.nix:3:5:

            2| { outputs = { self, nixpkgs }: {
            3|     packages.x86_64-linux.hello = let
             |     ^
            4|       foo = nixpkgs.lib.evalModules {
```

Fixes #4745

1 files changed, 4 insertions, 1 deletions

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 428adf4c2..1b9948eaf 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -1210,7 +1210,10 @@ static RegisterPrimOp primop_toPath({
 static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     if (evalSettings.pureEval)
-        throw EvalError("builtins.storePath' is not allowed in pure evaluation mode");
+        throw EvalError({
+            .msg = hintfmt("'%s' is not allowed in pure evaluation mode", "builtins.storePath"),
+            .errPos = pos
+        });
 
     PathSet context;
     Path path = state.checkSourcePath(state.coerceToPath(pos, *args[0], context));