Fix CI for build-llm-demo (#7308)

After https://github.com/pytorch/executorch/pull/7108 landed, it broke CI, in particular build-llm-demo test.

Here's an example:

https://github.com/pytorch/executorch/actions/runs/12301283060/job/34332275511#logs

It removed a folder than contained android artifacts.

Let's just revert back to 'rm -rf cmake-out'