Fix a JET error by narrowing the type signature of the create_worker(x, y) function (#175)

```
  ┌ create_worker(manager::Any, wconfig::Any) @ Distributed /workpath/Distributed.jl/src/cluster.jl:620
  │ no matching method found `kwcall(::NamedTuple{(:config,), <:Tuple{Any}}, ::Type{Distributed.Worker}, ::Any, ::Sockets.TCPSocket, ::Sockets.TCPSocket, ::Base.LibuvStream)` (1/2 union split): Core.kwcall(NamedTuple{(:config,)}(tuple(wconfig::Any)::Tuple{Any})::NamedTuple{(:config,), <:Tuple{Any}}, Distributed.Worker, (getfield(w::Core.Box, :contents)::Any).id::Any, r_s::Sockets.TCPSocket, w_s::Sockets.TCPSocket, manager::Union{Base.LibuvStream, Distributed.ClusterManager})
  └────────────────────
```

(cherry picked from commit 79b4ca8)

Author: DilumAluthge

src/cluster.jl

@@ -608,7 +608,7 @@ function launch_n_additional_processes(manager, frompid, fromconfig, cnt, launch
     end
 end
 
-function create_worker(manager, wconfig)
+function create_worker(manager::ClusterManager, wconfig::WorkerConfig)
     # only node 1 can add new nodes, since nobody else has the full list of address:port
     @assert LPROC.id == 1
     timeout = worker_timeout()