xonsh: 0.6.7 -> 0.6.8 (#42930)

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/yp41kipqmmsjl0ldi4b16z2590shv9dp-xonsh-0.6.8/bin/xon.sh had a zero exit code or showed the expected version
- /nix/store/yp41kipqmmsjl0ldi4b16z2590shv9dp-xonsh-0.6.8/bin/.xonsh-wrapped passed the binary check.
- /nix/store/yp41kipqmmsjl0ldi4b16z2590shv9dp-xonsh-0.6.8/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 0.6.8 with grep in /nix/store/yp41kipqmmsjl0ldi4b16z2590shv9dp-xonsh-0.6.8
- directory tree listing: https://gist.github.com/e53ad25422aa303572f4452029cb65b3
- du listing: https://gist.github.com/0c29b7c27c01e8d34986163ba00f1615