[3.15] gh-153631: Move to `macos-26` runner for iOS (GH-153632) (#153673)

[3.15] gh-153631: Move to `macos-26` runner for iOS (GH-153632) (#153673)

Switches iOS CI to use the macos-26 runner, and 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)

Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>