nixos: fix prompt garbling
nixos: fix prompt garbling

Our old prompt kept ketting cluttered up by a mysterious display of the
$PWD on the far right --- turns out this is part of the 'walter' prompt
that NixOS enables by default if you enable 'zsh' via
`programs.zsh.enable = true`.

listx/syscfg,listx/syscfg,listx/syscfg,listx/syscfg,listx/syscfg