Small tweak to run_in_sandbox to keep old versions of PHP happy.

classes/external/run_in_sandbox.php

@@ -185,7 +185,7 @@ public static function execute(
                 $paramsarray['jobeserver'] = $jobehostws;
             }
             // usecache set to false for these runs as we will never regrade them.
-            $runresult = $sandbox->execute($sourcecode, $language, $stdin, $filesarray, $paramsarray, usecache:false);
+            $runresult = $sandbox->execute($sourcecode, $language, $stdin, $filesarray, $paramsarray, false);
         } catch (Exception $ex) {
             throw new qtype_coderunner_exception("Attempt to run job failed with error {$ex->message}");
         }