nspawn, vmspawn, run0: add env var for turning off background tinting

Some people are just sad, sad lost souls who don't like even the tiniest
ray of color in their life. Let's add an env var knob for allowing them
to turn the background tinting off, to drive the last bit of color from
their life so that they can stay in their grey grey life.

src/nspawn/nspawn.c

@@ -5556,7 +5556,7 @@ static int run_container(
                                                 arg_console_width,
                                                 arg_console_height);
 
-                        if (!arg_background) {
+                        if (!arg_background && shall_tint_background()) {
                                 _cleanup_free_ char *bg = NULL;
 
                                 r = terminal_tint_color(220 /* blue */, &bg);