90dac235bb
NixOS has a pervasive dependency on bash. For instance, the X11 session script sources /etc/profile to get a reasonable environment. Thus we should not provide an option to disable bash. Also, enabling zsh no longer sets ‘users.defaultUserShell’ to zsh, to prevent a collision with bash's definition of the same option. (Changing the default shell is also something that should be left to the user.)