`IO.Process.spawn` leaks file handles on windows

Prerequisites

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Description

Using IO.Process.run or IO.Process.spawn directly causes 6 file handles to be created which are never closed.

Context

I wanted to fix #8547 but found another bug :-(

Steps to Reproduce

1. Install sysinternals handle (https://learn.microsoft.com/en-us/sysinternals/downloads/handle)
2. Run this repeatedly: #eval IO.Process.run { cmd := "cmd.exe" }
3. Run handle -p <pid of the worker process for the file that's being edited> -s

Expected behavior: Running the command multiple times shouldn't change the amount of open file handles, any file handles that were opened should have been closed after the process has terminated and is no longer being referenced.

Actual behavior: The amount of file handles goes up consistently by 6 each time.

Versions

Lean 4.21.0-nightly-2025-06-02

Impact

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Start with the IO.Process.run and IO.Process.spawn entry points and reproduce the leak using #eval IO.Process.run { cmd := "cmd.exe" } on Windows. Use Sysinternals Handle to compare open handles across repeated runs; done means handles opened for the terminated process are closed and the count no longer rises.