Change the Xcode version fallback behavior (#1092)
Change the Xcode version fallback behavior (#1092)

* Change the Xcode version fallback behavior

If a requested version of Xcode is not available on the CI machine, the
default Xcode version is automatically used instead and a warning
message is printed in the build log.

* Address code review feedback
bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration