Freeze cache keys include the absolute code-root path #12

ExecEnv.run keys the freeze cache on " ".join(cmd) + "|".join(self.pythonpath) plus the program (scriptorium/execute.py), and pythonpath is the code root resolved to an absolute path (render.prepare). The same book in another checkout (a git worktree, the main checkout after a merge, another machine) misses every cache entry and re-runs all code. For How to Train your Chatbot ch01 that is a 25-minute training run, and marginalia's frozen reader shows no outputs until it happens.

Wanted: key on the code root relative to the document's cwd, so .scriptorium/freeze.json is portable and can even be committed.