Symlinks in the sandbox are broken by the path to the buildstream cache containing symlinks
The specific use-case found is that during a build that tried to execute rst2man, it turned out to be a broken symlink.
The symlink turned out to be
../../../../../../../../../home/jonathanmaw/.cache/buildstream/build/debian-python-x9gp05kc/root/etc/alternatives/rst2man
instead of /etc/alternatives/rst2man
According to @tristanvb, the problem is utils.py:_relative_symlink_path() needs to be fixed, so that the realpath of the passed 'root' directory is used as well