Async cache upload

[terlar]

Right now the upload of the cache happens at the very end. I wonder if cache upload could be done async as things get built, kind of like what cachix can do. Now I think it adds some extra time to do the actual upload as it doesn't do anything else, just waits to complete the upload.

A non-blocking post-build hook perhaps or something watching the store, which I guess already is in place to know which paths to upload in the end.

[blaggacao]

My gut feeling tells me that this could cut out another 30% on cycle time.