builtin set_color: fix inconsistent output in --print-colors

fish -c 'set_color --print-colors --background blue' |xxd

does not print color enabling sequences but does print *disabling* sequences.
There's no need for that.

src/builtins/set_color.rs

@@ -78,7 +78,7 @@ fn print_colors(
             }
         }
         outp.write_wstr(color_name);
-        if !bg.is_none() {
+        if streams.out_is_terminal() && !bg.is_none() {
             // If we have a background, stop it after the color
             // or it goes to the end of the line and looks ugly.
             outp.write_command(ExitAttributeMode);