Use built in zsh function to get absolute path

[egaldamez]

Hello, I've recently started using this project in combination with direnv and noticed a small improvement. What's happening is the following

1. I cd into a directory with a .envrc file
2. Any command that's executed within this directory will always trigger an direnv: unloading hook

After some research, I found that this issue was related to set-env.zsh and the use of absolute_dir_path. In fact, I found a similar issue which was resolved:
halcyon/asdf-java#144

In summary, the fix uses zsh's built in functionality to get the absolute dir path. This makes absolute_dir_path obsolete 🙂.

[egaldamez]

resolves #105