fix(tvix/eval): Propagate catchables in NixAttrs::construct

Correctly propagate the case where the *key* of an attrset is a
Value::Catchable (eg { "${builtins.throw "c"}" = "b"; }) in
`NixAttrs::construct`, by converting the return type to
`Result<Result<Self, CatchableErrorKind>, ErrorKind>` (ugh!!) and
correctly handling that everywhere (including an `expect` in the
Deserialize impl for NixAttrs, since afaict this is impossible to hit
when deserializing from stuff like JSON).

Change-Id: Ic4bc611fbfdab27c0bd8a40759689a87c4004a17
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10786
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/vm/mod.rs

@@ -932,9 +932,11 @@ where
 
     fn run_attrset(&mut self, frame: &CallFrame, count: usize) -> EvalResult<()> {
         let attrs = NixAttrs::construct(count, self.stack.split_off(self.stack.len() - count * 2))
-            .with_span(frame, self)?;
+            .with_span(frame, self)?
+            .map(Value::attrs)
+            .into();
 
-        self.stack.push(Value::attrs(attrs));
+        self.stack.push(attrs);
         Ok(())
     }