Add --use-mock-keychain to Chrome args (dart-lang/browser_launcher#63)

Bug: https://github.com/dart-lang/sdk/issues/56519

Rolling the Chrome version in Dart CI to 128.0.6613.36
results in a system dialog requesting Chrome to use
the keychain in MacOS. This then results in timeouts due
to it being blocking. Instead, use a mock keychain.