Fix Zephyr CI cmake install failure by pinning to an existing PyPI version

[manuelcandales]

cmake==3.29 doesn't exist on PyPI anymore (currently existing versions are 3.29.0.1, 3.29.2, etc.), causing the test-arm-backend-zephyr trunk jobs to fail. Use 3.29.6 instead (the latest from the 3.29 series).

[pytorch-bot]

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❌ 1 New Failure, 27 Pending

As of commit 480c3c1 with merge base ad235f8 ():

NEW FAILURE - The following job has failed:

• trunk / test-cortex-m-e2e (mv2) / linux-job (gh)
  RuntimeError: Command docker exec -t b62a0ce75cba1e09edff6709e10314a79f24cd0ff4d2452fd43ac64cdeb45d30 /exec failed with exit code 92

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[Copilot]

Pull request overview

This PR updates the Zephyr CI-driven setup instructions to pin cmake to a PyPI version that exists, addressing Zephyr trunk job failures caused by cmake==3.29 no longer being available on PyPI.

Changes:

• Update Zephyr requirements install command to use cmake==3.29.6 instead of cmake==3.29.

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