eigen: 3.3.3 -> 3.3.4

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. 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 3.3.4 with grep in /nix/store/q5xnlq781hvj0skjlhnb1qw1k69pf048-eigen-3.3.4
- directory tree listing: https://gist.github.com/d83ce54c91cb6ecf3fd3eb94b666357f