Ensure problem-specifications is symlinked

[senekor]

This is especially relevant for the add-exercise workflow, which will suggest slugs of unimplemented exercises from problem-specifications only if the symlink exists. Otherwise no suggestions are made without any hint that something went wrong.

[ellnix]

Looks good to me. I don't see the reason for the
-L check (script seems to work fine anyway and might have the added benefit of ensuring that problem-specifications are on the correct commit), but there's probably an edge case I'm not thinking of.

[senekor]

The idea is that maybe you're testing some other commit, maybe you have some local changes in problem-specifications that you want to test how they interact with the Rust track. It would be annoying if running these scripts messed with those changes. So, only run the script if the symlink doesn't exist already.