Noisy errors in Jenkins logs: cannot clean up isotester workspace

Impact: meaningless errors in Jenkins logs, that make it more difficult than it should be to look for relevant information there, when investigating actual problems.

In https://gitlab.tails.boum.org/tails/jenkins-jobs/-/blob/master/macros/test_Tails_ISO.yaml we enable workspace-cleanup, which fails with errors such as jenkins[529]: Also: java.nio.file.FileSystemException: /var/lib/jenkins/workspace/test_Tails_ISO_feature-18875-kleopatra/submodules/chutney/lib/chutney/__pycache__/TorNet.cpython-39.pyc: Operation not permitted (in the logs of the Jenkins orchestrator service).

That's expected: there are files owned by root in the workspace, so jenkins cannot delete them.

I believe it's useless to try & delete the workspace with that Jenkins plugin: after a test suite run, we reboot the isotester, and while rebooting we have /etc/systemd/system/jenkins-slave.service.d/cleanup-workspace.conf which deletes the workspace (as root).

Proposal: drop workspace-cleanup for test_Tails_ISO_* jobs.

Cc @zen who's maintaining our Jenkins.

Note: For S11, this fits in:

• D.1 - Make Tails development process more efficient: Decreasing the amount of noise in the build system makes it easier for developers to find relevant information when investigating problems.