Add a possibility to always save the cache whether it is a hit w r t key or not.

The usecase is Flatpak builds. These always builds dependencies from scratch. These builds are stored in a cache. flatpak-builder knows whether the data ithis cache is valid or not.

caching this cache (sic!) saves a lot of build time, about 25 minutes in our opencpn case. Saving the cache is less than 15 seconds, so it's worth it. The problem is just that no key can be constructed which reflects if the cache is valid. Thus, the sane approach is to save the cache in each build, leaving it to flatpak-builder to decide what is usable.

But I see no clean way to do this. There are work arounds, but certainly nothing clean to be proud of.