Cache cleanup should not run for cancelled or failed jobs

[bigdaz]

If the Job execution is failed or cancelled, then it is quite possible that some required downloads and assets in the Gradle User Home will be incorrectly flagged as "unused" and removed during cache cleanup.

We should only run cache cleanup in the case where the build successfully runs to completion.

[bigdaz]

I suspect that cleanup of one or more transformed jars is one factor resulting #47, although it should not produce the Immutable workspace contents have been modified error, so I think there's likely a bug in Gradle 8.6 as well.