[llvm] [LIT] Workaround the 60 processed limit on Windows (PR #157759)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-m68k-linux-cross` running on `suse-gary-m68k-cross` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/27/builds/19658

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'lit :: windows-pools.py' FAILED ********************
Exit Code: 127

Command Output (stdout):
--
# RUN: at line 3
rm -Rf /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/utils/lit/tests/Output/windows-pools.py.tmp.dir && mkdir -p /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/utils/lit/tests/Output/windows-pools.py.tmp.dir
# executed command: rm -Rf /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/utils/lit/tests/Output/windows-pools.py.tmp.dir
# executed command: mkdir -p /var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/utils/lit/tests/Output/windows-pools.py.tmp.dir
# RUN: at line 4
python -c "for i in range(20): open(rf'/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/utils/lit/tests/Output/windows-pools.py.tmp.dir/file{i}.txt', 'w').write('RUN:')"
# executed command: python -c 'for i in range(20): open(rf'"'"'/var/lib/buildbot/workers/suse-gary-m68k-cross/clang-m68k-linux-cross/stage1/utils/lit/tests/Output/windows-pools.py.tmp.dir/file{i}.txt'"'"', '"'"'w'"'"').write('"'"'RUN:'"'"')'
# .---command stderr------------
# | 'python': command not found
# `-----------------------------
# error: command failed with exit status: 127

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/157759