* fix(autojump): fix autojump sourcing in nix (-darwin)

This plugin expects to source an sh/zsh file which sets up autojump, but
that is not done when running Nix on macos using nix-darwin. Looking at
the old value (/etc/profiles/per-user/$USER/bin/autojump), it points to a
binary file instead of the setup script.

* Use zsh file instead of sh