This fixes the warning message such as: "Warning: Lisp directory
'/tmp/guix-build-emacs-ert-runner-0.7.0-1.90b8fdd.drv-0/source': No such file
or directory" that would be printed upon running ert-runner.
* gnu/packages/emacs-xyz.scm (emacs-ert-runner)[phases]{install-executable}:
Filter out the temporary source directory from the paths composing the
EMACSLOADPATH variable used in the wrapper.