Move iOS app build to a separate workflow (#1010)

Summary:
This achieves 2 goals:

* To avoid building and testing iOS app on GitHub runner on every pull request, which is very expensive ($$$)
* This would also help shoumikhin when he want to add the release part here next after building and testing the app

Note that the workflow would still be run if the pull request updates the content of the app itself via `examples/demo-apps/**`.  This is probably the desired behavior to run this job only when it's needed.

```
on:
  push:
    branches:
      - main
      - release/*
  pull_request:
    paths:
      - .github/workflows/app-build.yml
      - build/test_ios_ci.sh
      - examples/demo-apps/**
```

Pull Request resolved: https://github.com/pytorch/executorch/pull/1010

Reviewed By: shoumikhin

Differential Revision: D50423235

Pulled By: huydhn

fbshipit-source-id: 05f6b6727a08b07c03290938a36b0fef13496fb2