feat: add repo directory env var

[jeff350]

Description

Fixes #146

This adds an environment variable XARGS_REPO_DIRECTORY so the temp directory can be referenced in scripts.

TODOs

Read the Gruntwork contribution guidelines.

• [x] Update the docs.
• [ ] Run the relevant tests successfully, including pre-commit checks.
• [ ] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [ ] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Added XARGS_REPO_DIRECTORY environment variable for path of temp directory

Migration Guide

N/A

[flinn]

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