Don't consider system buck2

[jathu]

Summary

What are the benefits of looking for the system buck2?

• I'm guessing only Meta engineers have buck2 pre-installed, and the internal version doesn't work with ET — it fails looking for a .buck2 file
• If someone really wants to use their system buck2, they can use the BUCK2 cmake arg

Test plan

CI

cc @larryliu0820

[pytorch-bot]

🔗 Helpful Links

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• trunk / test-models-linux-aarch64 (mobilebert, portable, linux.arm64.2xlarge) / linux-job (gh)
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