verify_exercises_in_docker: avoid version mismatch

[senekor]

The versions are pinned in the test runner. Hopefully they are not too
outdated, but they can easily be a few patch versions behind. If the
solution directory contains a lock file, these requirements often won't
be satisfied in the test runner.

[ellnix]

It seems fine, but did a conflict like this ever happen? If there's a case, I can't think of it. If it's prophylactic, that's fine.

Nevermind, this was the wrong PR to see first 😄 I see the other ones now.

[senekor]

It's a valid question, since the lock file might not be created in some (most?) workflows. I do sometimes have them lying around. For quick and dirty changes to exercises, I cd into the exercise directory, copy the example solution to lib.rs, make my changes, run cargo test, copy the changes back to the example solution, restore lib.rs. So, this did fix a real issue for me :)