gnu: emacs-ert-runner: Update to latest upstream commit.

The latest tag is many years old, and the latest upstream branch includes
important fixes such as displaying correctly the backtrace upon encountering
errors (see:
https://github.com/rejeep/ert-runner.el/commit/4b6e6cb81fb3cb9db60046f70256555a9ae97459).

* gnu/packages/emacs-xyz.scm (emacs-ert-runner): Update to the latest upstream
commit.
[origin]: Use GIT-FETCH instead of URL-FETCH to mitigate against GitHub's
dynamically generated tarballs.