xonsh: 0.6.4 -> 0.6.6

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

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

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/n2hvc59jjmqfww86pqz7w6hmr7wlklbw-xonsh-0.6.6/bin/xon.sh had a zero exit code or showed the expected version
- /nix/store/n2hvc59jjmqfww86pqz7w6hmr7wlklbw-xonsh-0.6.6/bin/.xonsh-wrapped passed the binary check.
- /nix/store/n2hvc59jjmqfww86pqz7w6hmr7wlklbw-xonsh-0.6.6/bin/xonsh passed the binary check.
- 2 of 3 passed binary check by having a zero exit code.
- 2 of 3 passed binary check by having the new version present in output.
- found 0.6.6 with grep in /nix/store/n2hvc59jjmqfww86pqz7w6hmr7wlklbw-xonsh-0.6.6
- directory tree listing: https://gist.github.com/dba795a63cf24871f010baf56dba68d0
- du listing: https://gist.github.com/afde349bcf27d995e595d895941de9de