Fix a JET error around matching methods for getindex(...)

```
┌ setup_launched_worker(manager::Distributed.SSHManager, wconfig::Distributed.WorkerConfig, launched_q::Vector{Int64}) @ Distributed /workpath/Distributed.jl/src/cluster.jl:563
│ no matching method found `getindex(::Nothing, ::Symbol)` (1/2 union split): cnt = ((wconfig::Distributed.WorkerConfig).environ::Union{Nothing, Dict})[:cpu_threads]
└────────────────────
```

Author: DilumAluthge

src/cluster.jl

@@ -567,7 +567,7 @@ function setup_launched_worker(manager, wconfig, launched_q)
     # same type. This is done by setting an appropriate value to `WorkerConfig.cnt`.
     cnt = something(wconfig.count, 1)
     if cnt === :auto
-        cnt = wconfig.environ[:cpu_threads]
+        cnt = (wconfig.environ::AbstractDict)[:cpu_threads]
     end
     cnt = cnt - 1   # Removing self from the requested number