Fixup a bug in the sync polling exec function

[NHDaly]

In the case where the response had to poll, there was a silly mistake
where we were reading the wrong field off the response.

Previously:

julia> RAI.exec(ctx, "nhd-test-1", "nhd-s", """
           2 + 2 * 2, "hi", 'a'
           """)
ERROR: KeyError: key "id" not found
Stacktrace:
 [1] getindex(h::Dict{String, JSON3.Object{Vector{UInt8}, Vector{UInt64}}}, key::String)
   @ Base ./dict.jl:481
 [2] exec(ctx::Context, database::String, engine::String, source::String; inputs::Nothing, readonly::Bool, kw::Base.Pairs{Symbol, Union{}, Tuple{}, NamedTuple{(), Tuple{}}})
   @ RAI ~/work/rai-sdk-julia/src/api.jl:403

This is now fixed:

julia> RAI.exec(ctx, "nhd-test-1", "nhd-s", """
           2 + 2 * 2, "hi", 'a', "hello", '2', 2.0
           """)
Dict{String, Any} with 4 entries:
  "metadata"    => JSON3.Object[{…
  "problems"    => Union{}[]
  "results"     => Pair{String, Arrow.Table}["/:output/Int64/String/…
  "transaction" => {…