Setting Time Zone for macOS 10.13 seems to be flaky. Let's reset it. If
it still fails to set it then exit the script execution.
Task-number: QTBUG-92174
Change-Id: I07b28d0511d8fc097a1da43b78de9a5131f6bfa7
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@qt.io>