picard-tools: 2.18.3 -> 2.18.4

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

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

These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.18.4 with grep in /nix/store/k62ym9i08d0w9sgmdjjfyy4lf2n9ig7p-picard-tools-2.18.4
- directory tree listing: https://gist.github.com/4d9e3062e701681d9b53033dc9511663