grml-zsh-config: 0.14.3 -> 0.15.0

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/grml-zsh-config/versions.

These checks were done:

- built on NixOS

- 0 of 0 passed binary check by having a zero exit code.
- 0 of 0 passed binary check by having the new version present in output.
- directory tree listing: https://gist.github.com/f301ffb4e2c7eb9a9e2a03bc7291a958
- du listing: https://gist.github.com/810ea8b2fb0bfc9f2bd5eee5e90fce2a