xmrig: 2.5.3 -> 2.6.1

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

This update was made based on information from https://repology.org/metapackage/xmrig/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 2.6.1 with grep in /nix/store/wsy4vdhmqcr0mydcgp77dncf2xq906kf-xmrig-2.6.1
- directory tree listing: https://gist.github.com/ff33b30777a95f9fa0654b4da0ae47c2