fix(tvix/eval): force value in builtins.typeOf

This prevents Nix programs to observe the "internal" type of thunks.
Possibly .type_of() is also an area of the runtime where we should panic
if "internal" would ever be returned.

Change-Id: I9f358044c48ad64896fb6a1b1a42f00a29efac00
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6539
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: wpcarro <wpcarro@gmail.com>
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -125,8 +125,10 @@ fn pure_builtins() -> Vec<Builtin> {
                 Ok(Value::String(format!("{}", value).into()))
             })
         }),
-        Builtin::new("typeOf", 1, |args, _| {
-            Ok(Value::String(args[0].type_of().into()))
+        Builtin::new("typeOf", 1, |args, vm| {
+            force!(vm, &args[0], value, {
+                Ok(Value::String(value.type_of().into()))
+            })
         }),
     ]
 }