fix: install iOS platform for Xcode

Run flutter build ipa \
Archiving com.adilhanney.saber...
Automatically signing iOS for device deployment using specified development team in Xcode project: 3DB8QX4Z23
Running pod install...                                             37.2s
Running Xcode build...
Xcode archive done.                                          7.3s
Failed to build iOS app
Uncategorized (Xcode): Unable to find a destination matching the provided destination specifier:
		{ generic:1, platform:iOS }

	Ineligible destinations for the "Runner" scheme:
		{ platform:iOS, id:dvtdevice-DVTiPhonePlaceholder-iphoneos:placeholder, name:Any iOS Device, error:iOS 18.0 is not installed. To use with Xcode, first download and install the platform }

════════════════════════════════════════════════════════════════════════════════
iOS 18.0 is not installed. To download and install the platform, open
Xcode, select Xcode > Settings > Components, and click the GET button for the
required platform.

For more information, please visit:
  https://developer.apple.com/documentation/xcode/installing-additional-simulator-runtimes
════════════════════════════════════════════════════════════════════════════════
Encountered error while archiving for device.

Author: adil192

.github/workflows/ios.yml

@@ -17,6 +17,7 @@ on:
 jobs:
   build-ios:
     name: Build for iOS
+    # TODO(adil192): Change to macos-latest after 30 August 2025 (https://github.com/actions/runner-images/issues/12520)
     runs-on: macos-15
     timeout-minutes: 60
     env:
@@ -35,7 +36,12 @@ jobs:
           flutter-version: 3.32.8
           cache: true
           cache-key: 'flutter-:os:-:channel:-:version:-:arch:'
-          architecture: x64
+
+      - name: Setup Xcode
+        run: |
+          xcode-select -s /Applications/Xcode_16.4.app
+          xcodebuild -runFirstLaunch
+          xcodebuild -downloadPlatform iOS -version 18.0
 
       - name: Install Codemagic CLI Tools
         run: pipx install codemagic-cli-tools