ci: build the example app for iOS Simulator on PRs

[mikemilla]

Why

No CI job compiled the Swift bridge in ios/. The Courier_iOS 5.8.6 → 5.9.0 bump (#68) went green, but courierDevice ?? CourierDevice() in putUserToken stopped compiling, because 5.9.0 made the zero-arg CourierDevice() init @MainActor. The fix had to be made by hand.

What

• New build-ios job (macos-15, Xcode 16.4, the same toolchain as courier-ios CI). It runs bundle exec pod update Courier_iOS in example-085/ios, checks that the resolved Courier_iOS matches the courier-react-native.podspec pin, then builds the example with xcodebuild -destination 'generic/platform=iOS Simulator' CODE_SIGNING_ALLOWED=NO.
  • Uses pod update Courier_iOS rather than pod install because bump PRs (bump-courier-ios-*) change the podspec without touching Podfile.lock.
  • Build-only: no secrets and no signing. A Debug simulator build skips JS bundling, so Env.tsx isn't needed.
  • Caches the CocoaPods download and spec caches. node_modules use the existing setup action's cache.
  • concurrency cancels superseded runs to save macOS minutes.
• Example Podfile: removed pod 'Courier_iOS', :git => …, :tag => '5.8.3'. It overrode the podspec pin, so the example never built against the version the library ships with. Refreshed Podfile.lock (now 5.8.6 from trunk).

Testing

• I couldn't run the RN build locally because the disk is too full. This PR's build-ios run is the first real check.
• I ran the same steps locally in the Flutter sibling PR. With the unfixed 5.9.0 bump, the build fails with call to main actor-isolated initializer … in a synchronous nonisolated context, which is the error this job is meant to catch.

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