Arguments: always render += instead of =

This allows to extend existing arguments

fixes #12941

library/Director/Objects/IcingaArguments.php

@@ -371,7 +371,7 @@ class IcingaArguments implements Iterator, Countable, IcingaConfigRenderer
 
             $args[$arg->argument_name] = $arg->toConfigString();
         }
-        return c::renderKeyValue('arguments', c::renderDictionary($args));
+        return c::renderKeyOperatorValue('arguments', '+=', c::renderDictionary($args));
     }
 
     public function __toString()

test/php/library/Director/Objects/rendered/command7.out

@@ -1,6 +1,6 @@
 object CheckCommand "___TEST___command" {
     command = [ PluginDir + "/bla" ]
-    arguments = {
+    arguments += {
         "-a" = {
             set_if = "$a$"
         }