enable clean_components by default: it saves a lot of local disk space, notably on virtual nodes;

--- a/src/Pure/Admin/isabelle_cronjob.scala	Sat Jan 28 22:31:40 2023 +0100
+++ b/src/Pure/Admin/isabelle_cronjob.scala	Sun Jan 29 16:49:17 2023 +0100
@@ -141,7 +141,7 @@
     history: Int = 0,
     history_base: String = "build_history_base",
     components_base: String = Components.dynamic_components_base,
-    clean_components: Boolean = false,
+    clean_components: Boolean = true,
     java_heap: String = "",
     options: String = "",
     args: String = "",