[3.13] gh-153631: Ensure iOS simulator is running before starting build (GH-153632...

[3.13] gh-153631: Ensure iOS simulator is running before starting build (GH-153632) (#153675)

Makes a small change to the iOS build script to improve build stability.
xcodebuild doesn't guarantee that the CoreSimulatorService is running before
starting a simulator. If the service isn't running, xcodebuild reports that no
simulators are available, and fails to start the test app. However, simctl
blocks until the simulator is available, and simctl is used to evaluate the
default simulator. So - the iOS build script now unconditionally determines the
default simulator, even if a specific simulator is requested.
(cherry picked from commit 2dc1a91af801ea896e21f6c6e6cc41f57e8268d9)