nixos/bash: fix root prompt

b4b67177b5 introduced a regression of
its own: the prompt would end with $ for all users, not with # for
root as it should.

nixos/modules/programs/bash/bash.nix

@@ -102,7 +102,7 @@ in
               # Emacs term mode doesn't support xterm title escape sequence (\e]0;)
               PS1="\n\[\033[$PROMPT_COLOR\][\u@\h:\w]\\$\[\033[0m\] "
             else
-              PS1="\n\[\033[$PROMPT_COLOR\][\[\e]0;\u@\h: \w\a\]\u@\h:\w]\$\[\033[0m\] "
+              PS1="\n\[\033[$PROMPT_COLOR\][\[\e]0;\u@\h: \w\a\]\u@\h:\w]\\$\[\033[0m\] "
             fi
             if test "$TERM" = "xterm"; then
               PS1="\[\033]2;\h:\u:\w\007\]$PS1"