Reviewing python PRs is broken when `nixpkgs-review` is made available via direnv

[Stebalien]

The issue happens when nixpkgs-review (or any python package) is installed in the current shell via direnv. E.g., echo "use flake" > .envrc in the nixpkgs repo.

Here's what I think is happening:

1. Direnv sets the PYTHONPATH when it makes nixkgs-review available in the current shell.
2. nixpkgs-review pr #SOME_PYTHON_PR sets-up a nix-shell with a different PYTHONPATH (one that contains the python package to review).
3. nixpkgs-review changes directory to ~/.cache/nixpkgs-review/... (at some point).
4. nixpkgs-review opens a nix shell.
5. The nix shell displays a prompt which runs the direnv hook.
6. The direnv hook notices that the current directory doesn't match DIRENV_DIR, so it unloads the environment, unsetting PYTHONPATH.

It's possible to work around this by setting programs.direnv.loadInNixShell to false, but that's not the default behavior and debugging this is non-trivial.

IMO, the right way to fix this is for nixpkgs-review to clean its environment before and change to the ~/.cache directory entering the nix shell.

It would also be possible to fix this in direnv by checking if we're in a nixpkgs-review session here, but that won't help anyone using direnv via other means.