nixos/switch-to-configuration: Allow not filtering units

nixos/doc/manual/development/what-happens-during-a-system-switch.chapter.md

···
 - Inspect what changed during these actions and print units that failed and
   that were newly started
 
+By default, some units are filtered from the outputs to make it less spammy.
+This can be disabled for development or testing by setting the environment variable
+`STC_DISPLAY_ALL_UNITS=1`
+
 Most of these actions are either self-explaining but some of them have to do
 with our units or the activation script. For this reason, these topics are
 explained in the next sections.

nixos/modules/system/activation/switch-to-configuration.pl

···
                     $units_to_start{$unit} = 1;
                     record_unit($start_list_file, $unit);
                     # Don't spam the user with target units that always get started.
-                    $units_to_filter{$unit} = 1;
+                    if (($ENV{"STC_DISPLAY_ALL_UNITS"} // "") ne "1") {
+                        $units_to_filter{$unit} = 1;
+                    }
                 }
             }