Try to fix test_model.sh (#5361)

Summary:
Trying to fix failures such as https://github.com/pytorch/executorch/actions/runs/10855583772/job/30128512970

For the time missing operator error is repro'ed on my Mac, I see cmake-out/ was not cleaned up and merged.yaml was missing linear.out because it was super old.

Now without deep understanding of how CI job workers cache previous runs, I'm refactoring `build_cmake_executor_runner` in test_model.sh to make sure it always build clean.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5361

Reviewed By: swolchok

Differential Revision: D62665029

Pulled By: larryliu0820

fbshipit-source-id: 9c15c6581079b544084ae53979c995bc83cc46f9