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

[DilumAluthge]

This is a forward-port of JuliaLang/Distributed.jl#175 (JuliaLang/Distributed.jl@d65a996).

  ┌ 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 d65a9960ae358806312c1adf35f45c62f02caf0b)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.53%. Comparing base (fbca46f) to head (a04809d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
+ Coverage   87.39%   87.53%   +0.14%     
==========================================
  Files          11       11              
  Lines        2086     2086              
==========================================
+ Hits         1823     1826       +3     
+ Misses        263      260       -3     

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