Fix a JET error around matching methods for shell_escape_wincmd(...) (#174)

```
┌  @ Distributed /workpath/Distributed.jl/src/managers.jl:323
│ no matching method found `shell_escape_wincmd(::Nothing)` (1/2 union split): remotecmd = Distributed.shell_escape_wincmd(Distributed.escape_microsoft_c_args(tuple(exename::Any)::Tuple{Any}, exeflags::Cmd...)::Union{Nothing, String})
└────────────────────
```

(cherry picked from commit 75c559c)

Author: DilumAluthge

src/managers.jl

@@ -320,7 +320,7 @@ function launch_on_machine(manager::SSHManager, machine::AbstractString, cnt, pa
 
         any(c -> c == '"', exename) && throw(ArgumentError("invalid exename"))
 
-        remotecmd = shell_escape_wincmd(escape_microsoft_c_args(exename, exeflags...))
+        remotecmd = shell_escape_wincmd(escape_microsoft_c_args(exename, exeflags...)::AbstractString)
         # change working directory
         if dir !== nothing && dir != ""
             any(c -> c == '"', dir) && throw(ArgumentError("invalid dir"))