refactor(tvix/eval): use EvalIO::path_exists for the builtin

Change-Id: I49822ce30137777865e7370ee86666636e277b35
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7573
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/builtins/impure.rs

@@ -28,7 +28,8 @@ mod impure_builtins {
 
     #[builtin("pathExists")]
     fn builtin_path_exists(vm: &mut VM, s: Value) -> Result<Value, ErrorKind> {
-        Ok(coerce_value_to_path(&s, vm)?.exists().into())
+        let path = coerce_value_to_path(&s, vm)?;
+        vm.io().path_exists(path).map(Value::Bool)
     }
 
     #[builtin("readDir")]