Make it clear when linux installers are blocked on one particular machine

From release retrospective: https://github.com/adoptium/adoptium/issues/104#issuecomment-1036065992

At the moment if the machines for one platform is unavailable the builds stall in a non-obvious way (I found blueocean was the best place to check what had stalled). It would be much better if we can get earlier identification/notification when this situation occurs so that releasing is not delayed longer than is necessary.

Start with the linked release retrospective and inspect the installer build flow for cases where machines for one platform are unavailable. Done means unavailable machines are identified and notifications arrive early enough that a release does not silently stall; the issue does not name a file or test to run.

Written by the indexing model from the issue text.