Lake build should be runnable from subdirectory

lake build expects to be run from the top-level directory of a project. In contrast, cargo build can be run from anywhere within the project, including subdirectories. cargo build figures out what the root directory is, and it would be nice for lake to be able to do the same.

One issue that I had not thought about, but Mac mentioned to me in private communication is that:

changing directories within Lake would not change the version of Lake itself (i.e., it would not naturally respect the lean-toolchain of the switched to directory).

I don't have a clean solution for this, but I open an issue regardless as a feature request.

Start by tracing how lake build discovers the project root and how it selects the lean-toolchain. Compare that behavior with the issue's cargo build example; done means builds can be launched from a project subdirectory without silently changing the Lake version used for the target directory.