provide an environment variable with the temp directory name

**Describe the solution you'd like**
add a XARGS_REPO_DIRECTORY variable to access the path of the temporary directory

**Describe alternatives you've considered**
I can't find an easy way to access the path

**Additional context**
I came across this issue while trying to use git-xargs to appy copier templates across a large number of repos. The issue is that copier requires both the template and target directory to be specified. Here copier's target directory needs to be git-args temp directory