Fix Errors when running make clean.
make clean fails to find Cargo.toml files.
The command outputs multiple lines similar to
error: manifest path `cookbook/pkgutils/Cargo.toml` does not exist. The cause seems to be that the failing commands are run from the wrong working directory. See Makefile.
Solution: By changing back into the repository root before running the failing commands, they are able to execute successfully.
Fixes: #1282 (closed)