Also save cache on job failure

[TimJentzsch]

Unfortunately, the cache action only saves the cache if the workflow succeeded.
For cargo, this doesn't make much sense as e.g. a failing test doesn't influence the build artifacts.

The difficulty here is that the action does not provide any option to configure this.
We could use the https://github.com/actions/cache/tree/main/save action to manually save the cache, but then we can't put it at the end of the job, since compose actions don't have a way to set a post action to my knowledge...