This starts emacs in the local copy of the .emacs.d directory we are currently in, by creating a new `sandbox` directory as home directory, linking the current directory as `.emacs.d` into there, and starting emacs.
For development purposes.
Compilation is part of the testing.
Inspired by https://nullprogram.com/blog/2020/01/22/ and fancy to use make :) Yay make!