Stub header is not found when a specified solution is runned with -s

[mraron]

Using the latest (ppa) version.
Minimal reproducible example:
mre.zip

Be at the task root. Using task-maker-rust compiles solution.cpp just fine. But running via task-maker-rust -s sol/solution.cpp produces compilation errors about missing header. Being at the sol dir task-maker-rust -s solution.cpp is fine as well.