mate.mate-icon-theme: 1.20.0 -> 1.20.1 (#42237)

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

This update was made based on information from https://repology.org/metapackage/mate-icon-theme/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.
- found 1.20.1 with grep in /nix/store/sqb3ppyysd75gan41g7r3466gc01sxiz-mate-icon-theme-1.20.1
- directory tree listing: https://gist.github.com/a482def5741d5681816fc5e7c8d14264
- du listing: https://gist.github.com/10c651e4bf3c2f51c245c47429b3bb45