Check for throw while evaluating throw message

This is a small tweak to the logic added in cl/1511 to detect explicit
throws when printing stack traces. Now when deciding whether to print
"caused by explicit throw", it checks not only that the error is a
ThrowError and that we are in a throw, but also that the ThrowError was
thrown by *this* throw, and not by another while in the process of
evaluating this throw's operand.

It turns this:

```
let
  set = {
    inner = throw "nested throw";
  };
in
throw set.inner

error:
       … caused by explicit throw
         at /pwd/in.nix:6:1:
            5| in
            6| throw set.inner
             | ^
            7|

       … while evaluating the attribute 'inner'
         at /pwd/in.nix:3:5:
            2|   set = {
            3|     inner = throw "nested throw";
             |     ^
            4|   };

       … caused by explicit throw
         at /pwd/in.nix:3:13:
            2|   set = {
            3|     inner = throw "nested throw";
             |             ^
            4|   };

       error: nested throw
```

into this:

```
error:
       … while calling the 'throw' builtin
         at /pwd/in.nix:6:1:
            5| in
            6| throw set.inner
             | ^
            7|

       … while evaluating the attribute 'inner'
         at /pwd/in.nix:3:5:
            2|   set = {
            3|     inner = throw "nested throw";
             |     ^
            4|   };

       … caused by explicit throw
         at /pwd/in.nix:3:13:
            2|   set = {
            3|     inner = throw "nested throw";
             |             ^
            4|   };

       error: nested throw
```

Notice the difference in the top context frame. Before it incorrectly
attributed the throw error to the both throws instead of just the one
that actually threw.

Change-Id: If3b0b8311f1ae2ff1471e260fe59d9166a6a6964

lix/libexpr/eval.cc

@@ -1206,7 +1206,7 @@ Value EvalState::callFunction(Value & fun, std::span<Value> args, const PosIdx p
                 } catch (ThrownError & e) {
                     // Distinguish between an error that simply happened while "throw"
                     // was being evaluated and an explicit thrown error.
-                    if (fn->name == "throw") {
+                    if (fn->name == "throw" && !e.hasTrace()) {
                         e.addTrace(ctx.positions[pos], "caused by explicit %s", "throw");
                     } else {
                         e.addTrace(ctx.positions[pos], "while calling the '%s' builtin", fn->name);

tests/functional2/lang/select-err/eval-fail-2.err.exp

@@ -0,0 +1,23 @@
+error:
+       … while calling the 'throw' builtin
+         at /pwd/in.nix:6:1:
+            5| in
+            6| throw set.inner
+             | ^
+            7|
+
+       … while evaluating the attribute 'inner'
+         at /pwd/in.nix:3:5:
+            2|   set = {
+            3|     inner = throw "nested throw";
+             |     ^
+            4|   };
+
+       … caused by explicit throw
+         at /pwd/in.nix:3:13:
+            2|   set = {
+            3|     inner = throw "nested throw";
+             |             ^
+            4|   };
+
+       error: nested throw

tests/functional2/lang/select-err/in-2.nix

@@ -0,0 +1,6 @@
+let
+  set = {
+    inner = throw "nested throw";
+  };
+in
+throw set.inner