Restore explicit iOS simulator destination in CI workflow

Restored `IOS_SIM_DESTINATION` environment variable in `.github/workflows/scripts-ios.yml` to force usage of "iPhone 15" instead of relying on auto-detection which led to CI failures with "iPhone 16 Pro".

Author: google-labs-jules[bot]

.github/workflows/scripts-ios.yml

@@ -119,6 +119,7 @@ jobs:
       - name: Run iOS UI screenshot tests
         env:
           ARTIFACTS_DIR: ${{ github.workspace }}/artifacts
+          IOS_SIM_DESTINATION: "platform=iOS Simulator,name=iPhone 15,OS=latest"
         run: |
           set -euo pipefail
           mkdir -p "${ARTIFACTS_DIR}"