fix(tvix/eval): ? operator should use OpAttrsOrNotFound

Using `OpAttrSelect`, the ? operator will fail when encountering a
nested value that is not an attribute set.

This however breaks valid code, such as:

  { bs = 42; } ? bs.a.b

The fix is simply to use the same operator used in the `or` statement,
which leaves a sentinal on the stack if a field is not found or the
value is not an attribute set.

Change-Id: Ib28fc8a96e6d592b4cdbc3e65ba129ad8faecd66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6211
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/compiler.rs

@@ -568,7 +568,7 @@ impl Compiler {
                 self.compile_with_literal_ident(next)?;
 
                 for fragment in fragments.into_iter().rev() {
-                    self.chunk.push_op(OpCode::OpAttrsSelect);
+                    self.chunk.push_op(OpCode::OpAttrOrNotFound);
                     self.compile_with_literal_ident(fragment)?;
                 }
 

tvix/eval/src/tests/tvix_tests/eval-okay-contains-nested-non-set.exp

@@ -0,0 +1 @@
+false

tvix/eval/src/tests/tvix_tests/eval-okay-contains-nested-non-set.nix

@@ -0,0 +1,3 @@
+# ? operator should work even if encountering a non-set value on the
+# walk
+{ a.b = 42; } ? a.b.c