mate.libmatemixer: 1.20.0 -> 1.20.1

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

This update was made based on information from https://repology.org/metapackage/libmatemixer/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/11ay5byrlalp3z70d2nn3byf9i99m12p-libmatemixer-1.20.1
- directory tree listing: https://gist.github.com/6de4fa8f22cf5e05683ba4f03784a1f0
- du listing: https://gist.github.com/f63fa290542df670f00cabe5c6a7c5ca