[CI/Build] force writing version file

[dtrifiro]

#8772 migrated static metadata to pyproject.toml, but had the unintended consequence of not creating the vllm/_version.py at build time.

[github-actions]

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[dtrifiro]

@simon-mo this is a follow up o #8772, which we urgently need to unbreak the build

[youkaichao]

confirmed it works from the neuron test. thanks for the fix!