[worker] Simplify tests (#3285)

# Why

The worker integration tests were slow and required platform-specific CI runners (macOS for iOS, Linux for Android) because they attempted to build real React Native apps. This made the CI pipeline complex, slow, and resource-intensive.

# How

- **Replace full-build tests with fast mocked tests**: Instead of running actual Android/iOS builds, we now mock `@expo/build-tools` at the library boundary. This lets us test the worker's WebSocket communication, job dispatch, state sync, abort handling, and hanging worker detection without the overhead of real builds.

- **Add new `jobExecution.test.ts`**: A new integration test that verifies the worker accepts jobs, executes them (via mocked builders), and returns success/error responses correctly.

- **Update existing tests**: `abortBuild.test.ts`, `stateSync.test.ts`, and `possiblyHangingWorker.test.ts` now mock `@expo/build-tools` instead of the worker's internal `build.ts` module, allowing more of the real code path to execute.

- **Simplify CI**: Consolidated the separate `worker-android-checks` and `worker-ios-checks` jobs into a single `worker-checks` job running on `ubuntu-latest`. Removed dependencies on `direnv`, `setup-gcloud`, and platform-specific scripts.

- **Cleanup**: Deleted unused files including `__mocks__/build.ts`, macOS-specific Jest config, CI shell scripts, and the `direnv` internal action.

The build-build part is going to be tested in system tests running in CI on `main`.

# Test Plan

cd packages/worker
yarn test:integrationAll 19 tests pass in ~6 seconds (down from minutes with real builds).

```bash
cd packages/worker
yarn test:integration
```

All 19 tests pass in ~6 seconds (down from minutes with real builds).