A CI system like TeamCity will give you that sort of thing for free though, without needing an Earthfile. It just dedupes builds at the same commit, and keeps checkout directories between builds on the agents, so if the build tool has a cache then it's automatically reused. Works great.

Given that it's hard to understand what exactly this is for? Build systems normally try to avoid redundant work for local purposes anyway. Is it some hack for companies that insist on resetting build environments to zero on every single build, and then decide they want caching back? If so, why not just ... let the builds take place in directories containing previous builds, and fix the problems? That's what we do at my company and it works great (using gradle), there are only rarely problems with unclean builds and it's easy to rerun with a clean checkout if that's suspected to be an issue (the bugs are mostly in our own code which does a layer of caching above the build system anyway).