Jenkinsfile: work around flaky apt cache
The apt cache on our server seems to be flaky. Florian is notified of the issue (INFRA-25), but in the mean time, skip the cache and use Ubuntu's archive server directly.
The apt cache on our server seems to be flaky. Florian is notified of the issue (INFRA-25), but in the mean time, skip the cache and use Ubuntu's archive server directly.