nixos/bash: use simple prompt for dumb terminals

nixos/modules/programs/bash/bash.nix

@@ -90,6 +90,7 @@ in
 
       promptInit = mkOption {
         default = ''
+          if test "$TERM" != "dumb"; then
             # Provide a nice prompt.
             PROMPT_COLOR="1;31m"
             let $UID && PROMPT_COLOR="1;32m"
@@ -97,6 +98,7 @@ in
             if test "$TERM" = "xterm"; then
               PS1="\[\033]2;\h:\u:\w\007\]$PS1"
             fi
+          fi
         '';
         description = ''
           Shell script code used to initialise the bash prompt.