focuswriter: 1.6.12 -> 1.6.13 (#41567)

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

This update was made based on information from https://repology.org/metapackage/focuswriter/versions.

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/rj9lzdc106vkfwgw7f055ki4avyjzxg2-focuswriter-1.6.13/bin/focuswriter had a zero exit code or showed the expected version
- 0 of 1 passed binary check by having a zero exit code.
- 0 of 1 passed binary check by having the new version present in output.
- found 1.6.13 with grep in /nix/store/rj9lzdc106vkfwgw7f055ki4avyjzxg2-focuswriter-1.6.13
- directory tree listing: https://gist.github.com/db4b900533de6fbcd9a1a49a687360bb
- du listing: https://gist.github.com/4a9ea745f0fc337735be9262721a0a12