proto files are not resolved correctly when compiled from repos from other workspaces

[tiziano88]

We currently use this as a hack to find our way to the protos (and other non-rust files) from the root of our repo:

oak/.cargo/config.toml

Lines 30 to 33 in dbbe410

	# A cargo environment variable holding the root directory of our repo.
	# We can use WORKSPACE_ROOT instead of `std::env::current_dir()` to navigate in the repo.
	[env]
	WORKSPACE_ROOT = { value = "", relative = true }

unfortunately this does not work when another workspace / repo depends on one of our crates ,since that env variable will be set relative to that repo, and therefore the proto messages won't be found there

cc @dzmitry-huba @dingelish

[tiziano88]

cc @agl

[tiziano88]

possibly relevant too: https://doc.rust-lang.org/cargo/reference/manifest.html#the-exclude-and-include-fields