fix: install iOS platform for Xcode

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:
@@ -37,6 +38,9 @@ jobs:
           cache-key: 'flutter-:os:-:channel:-:version:-:arch:'
           architecture: x64
 
+      - name: Install iOS platform for Xcode
+        run: xcodebuild -downloadPlatform iOS -version 18.0
+
       - name: Install Codemagic CLI Tools
         run: pipx install codemagic-cli-tools