Consider [this execution](https://github.com/google/zerocopy/actions/runs/10976322510/job/30476954004?pr=1723). First, the cache [detects a cache hit](https://github.com/google/zerocopy/actions/runs/10976322510/job/30476954004?pr=1723#step:3:31). Next, it still [executes the "Download dependencies" step](https://github.com/google/zerocopy/actions/runs/10976322510/job/30476954004?pr=1723#step:4:1), which takes 1m55s. Finally, since there was a cache hit, it decides [not to save any of this work](https://github.com/google/zerocopy/actions/runs/10976322510/job/30476954004?pr=1723#step:7:3). Instead, when there's a cache hit, we should just abort immediately so we don't block CI needlessly.