Option to only use exact cache hit

[Nemo157]

On docs.rs we've recently run into disk space issues in CI. Debugging it I noticed that we had a very large cache (3GB compressed), looking at the lists of files there were many duplicate versions of crates included in it. I believe the main cause was having multiple large sets of dependency updates merged within a week, so the previous versions were not being cleaned when restoring a partially-matched cache. After setting an explicit prefix-key to purge the cache it dropped down to just 1.2GB.

We don't do many frequent dependency updates, instead batching them up, so the partial cache matches aren't generally that useful for us; having the cache not carrying the extra weight for a week after the update is more important.

[Nemo157]

Another idea I had to solve our issue was an option to not restore the cache. We could then pass a conditional expression that enabled this just for our dependency updates, so other PRs that modify versions would still use a partial cache and just the mass update PRs would flush the cache out to a new state.