Allow to override builtins with user env

[isubasinghe]

There are scenarios where Eval is called with keywords being present in the expression itself.
These can fail because of the type checking.

func nameclash() {
  env := make[string]any{
    "foo": 1,
    "split": 2
  }
  output, err := expr.Eval("foo + split", env)
  if err != nil {
    // currently this branch will be reached
    return;
  }
}

[antonmedv]

Yes, I now I think it is clear what it is better to allow override built-in function with any env variables.

[agilgur5]

For posterity, this was filed as a result of argoproj/argo-workflows#12037, which was a regression from previous behavior. Since the new built-ins and type checks were released in a minor, the previous behavior should remain, which is what skewed the decision in favor.