The fetching from 3rd party repos has become very flaky.
To avoid mirrors throttling us and other flaky issues cache
the dependencies and set -o to enable offline mode.
This seems to also speed up the step considerably.
Pick-to: 6.9 6.8 6.5
Change-Id: I01663e60baa44ab828d8880ab8b346034a50b60e
Reviewed-by: Simo Fält <simo.falt@qt.io>