Future support for shared compile cache

[kentfredric]

As-is, things like ccache don't produce enough leverage, and trying to test/build crates at scale is painful, with literally every crate having to be recompiled every time.

There may be options involving RUST_TARGET_DIR , to somewhere like /var/tmp/cargo/, but there's still issues here.

1. Concurrent compiles are blocked by this
2. It bloats quickly
3. We'd need some management tools to clean it up automatically: [1][2]
4. It could potentially mess up how we currently do build tooling :/