Notify build system on rebuild

[Profpatsch]

When you use e.g. a Makefile for building your project’s dependencies, and you change your shell.nix, you’d want your build system to pick up the change.

There is no good synchronous solution here I think, since you can either

• wrap every command in nix-shell and have the overhead
• have some kind of symlink directory in your build directory that your build system depends on, which links to all the tools (maybe?)
• make lorri touch a file once it finishes building, leading to invalidation of the build system

I feel like the last point is pretty easy to implement, you could tell lorri to touch e.g. .build/latest-lorri-eval and then depend on it in your makefile. It is racy of course, but better than if the build system never notices that something changed.

[Profpatsch]

Maybe there could even be a way to block the build system until the rebuild is finished? That would trade speed for correctness, but if you e.g. changed one of your code generation tools you really want to wait on it to be available.