Allow global pipenv installation + let user install venv manually

[laermannjan]

Problem

1. Currently, if you have python and pipenv in ~/.config/rtx/config.toml this plugin will create a pipenv venv in every directory you navigate to.

2. If you configure your local .rtx.toml according to the readme, the automatic pipenv venv installation would pickup the globally available python, not the local one (I guess due to the parallel execution by rtx).

Suggested Fix

1. skip pipenv setup when no Pipfile is found in current directory
2. let the user install the pipenv venv manually after rtx has done its thing