iperf2: 2.0.11 -> 2.0.12

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

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

These checks were done:

- built on NixOS
- /nix/store/c7p3b3b353pssma0cg1wg78pfs6x2is3-iperf-2.0.12/bin/iperf passed the binary check.
- 1 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 2.0.12 with grep in /nix/store/c7p3b3b353pssma0cg1wg78pfs6x2is3-iperf-2.0.12
- directory tree listing: https://gist.github.com/457110ea6db3cd3dbd6c9bc7ff82477a
- du listing: https://gist.github.com/86655ffd63846a5d3a473393af8e4da9