Flux `sleep` is evaluated during task creation

__Steps to reproduce:__
List the minimal actions needed to reproduce the behavior.

1. Create a new task that uses flux's `sleep` function:

```
from(bucket: "...") |> range(...) |> sleep(duration: 10s)
```

__Expected behavior:__
The task should be created without sleeping.

__Actual behavior:__
The task creation hangs for the given sleep duration because the script is evaluated on creation.

Start by reproducing task creation with a Flux script containing `sleep(duration: 10s)` and confirm that creation blocks. Trace where the task script is evaluated during creation; done means creation returns without waiting for the sleep duration while task execution still handles the query correctly.

Written by the indexing model from the issue text.