ci: Avoid forcing iOS SDK for DemoApp build

The DemoApp scheme embeds a watchOS app. Passing -sdk iphonesimulator makes Xcode 15.4 resolve parts of the embedded watch build through iOS simulator package product paths, which fails at link time.

Let the destination select the SDK for the mixed-platform scheme while keeping explicit SDKs for the package-only jobs.

Refs EME-1168
Co-Authored-By: Codex <noreply@openai.com>

Author: cameroncooke

.github/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Xcode select
         run: sudo xcode-select -s '/Applications/Xcode_15.4.app/Contents/Developer'
       - name: Build TestApp
-        run: cd Examples && xcodebuild build -scheme DemoApp -sdk iphonesimulator -destination 'generic/platform=iOS Simulator' -project DemoApp/DemoApp.xcodeproj
+        run: cd Examples && xcodebuild build -scheme DemoApp -destination 'generic/platform=iOS Simulator' -project DemoApp/DemoApp.xcodeproj
       - name: Build Snapshotting
         run: xcodebuild build -scheme Snapshotting -sdk iphonesimulator -destination 'generic/platform=iOS Simulator'
       - name: Build SnapshottingTests