fix malloc-stack CI failure

[vtjnash]

We see Windows CI fail often here, and we don't actually need this memory--only the first thread should initialize it. (indeed, we were temporarily corrupting these data-structures in the process by reinitializing them, though that was not typically detectable.)

From worker 7:  Exception: EXCEPTION_ACCESS_VIOLATION at 0x7ffa26726e87 -- jl_makecontext at C:/workdir/src\win32_ucontext.c:67
From worker 7:  in expression starting at C:\buildkite-agent\builds\win2k22-amdci6-6\julialang\julia-master\julia-ceffaee345\share\julia\test\ccall.jl:1066
From worker 7:  jl_makecontext at C:/workdir/src\win32_ucontext.c:67
From worker 7:  jl_install_thread_signal_handler at C:/workdir/src\signals-win.c:490
From worker 7:  jl_init_root_task at C:/workdir/src\task.c:1568
From worker 7:  ijl_adopt_thread at C:/workdir/src\threading.c:414
From worker 7:  unknown function (ip: 000001d791a58969)
From worker 7:  uv__queue_work at /workspace/srcdir/libuv\src\threadpool.c:305
From worker 7:  worker at /workspace/srcdir/libuv\src\threadpool.c:122
From worker 7:  uv__thread_start at /workspace/srcdir/libuv\src/win\thread.c:111
From worker 7:  beginthreadex at C:\Windows\System32\msvcrt.dll (unknown line)
From worker 7:  endthreadex at C:\Windows\System32\msvcrt.dll (unknown line)
From worker 7:  BaseThreadInitThunk at C:\Windows\System32\KERNEL32.DLL (unknown line)
From worker 7:  RtlUserThreadStart at C:\Windows\SYSTEM32\ntdll.dll (unknown line)
From worker 7:  Allocations: 352796158 (Pool: 352389694; Big: 406464); GC: 722