Fix install ET without pybind

[silverguo]

Fix bash script, this prevents us from passing false to ENABLE_ET_PYBIND.
Example cmd:
./torchchat/utils/scripts/install_et.sh false

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchchat/1509

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⏳ 1 Pending, 1 Unrelated Failure

As of commit f17014f with merge base e5cf6e5 ():

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-torchao-aoti-experimental (macos-14-xlarge) (gh) (trunk failure)
  Process completed with exit code 134.

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