Jenkins workspaces for disabled jobs are not removed from ISO builders

Originally created by @intrigeri on #9350 (Redmine)

See /var/lib/jenkins/workspace on isobuilder{1,2}.lizard. This will grow out of control once we build all active branches. There’s surely a Jenkins config pref or plugin to handle that. And worst case, we can simply cron the removal of all workspaces that are not being used by a currently running build (the workspace directory is kept open for the entire build duration, as soon as build-tails is started, apparently — but perhaps there’ll be race conditions wrt. what happens before or after, so it would probably be more robust to implement this in another way).

Parent Task: #6196 (closed)

Related issues

• Related to #8912 (closed)
• Blocks #8658 (closed)