Source environment relatively from script path in bin/setup

[CasEbb]

When following the installation instructions from the readme, running bin/setup yields a warning magento.env file not found. This is because the environment is sourced from ../env/magento.env, and assuming we're running the script in $PWD, it is trying to source the env from $PWD/../env/magento.env, while the file is in $PWD/env/magento.env (assuming we didn't cd into bin/).

This PR changes bin/setup to prepend the paths with the script dir so it always finds magento.env regardless of the working directory. This is also probably a more durable solution than the one proposed in #1385.

[what-the-diff]

PR Summary

• Dynamic Path Update for "magento.env" File
  This update introduces a change in how we locate the "magento.env" file. Instead of using a fixed path, we have made it dynamically determined. This enhances its portability and adaptability, essentially making it easier for our system to find and use this file, regardless of where it's stored.