task_group: make the whole task to be part of the flow

Initially only the contents of the tasks field was passed to the flow.

The problem is that with an embedding references with top level fields won't be resolved properly in tasks.

Ex:

group: {
    #TaskGroup

    result: tasks.one.stdout

    tasks: one: exec.#Run & {
        stdout: string
        ...
    }

    tasks: two: exec.#Run & {
        stdin: result
        ...
    }
}

Bad example, but if we consider only the tasks field in the flow tasks.two.stdin will never be resolved because we "reroot" the flow value.