[libc++] LLVM-libc CI configuration is brittle because it downloads Linux kernel headers

In:
https://github.com/llvm/llvm-project/blob/8f1bac0975c74f4379d9cc402fbebc8d5c7c0dba/libcxx/utils/ci/run-buildbot#L460-L464

It looks like these headers are not available anymore, so this fails:

```
curl -L https://cdn.kernel.org/pub/linux/kernel/v7.x/linux-7.0.1.tar.xz --output /home/gha/actions-runner/_work/llvm-project/llvm-project/build/generic-llvm-libc/linux.tar.xz
```

See https://github.com/llvm/llvm-project/actions/runs/28604072312/job/84831706962?pr=207215