ensure permissions when starting build task (e.g., due to misconfigured client);

--- a/src/Pure/Build/build_manager.scala	Wed Jun 05 17:27:13 2024 +0200
+++ b/src/Pure/Build/build_manager.scala	Wed Jun 05 17:41:16 2024 +0200
@@ -633,6 +633,8 @@
           val number = _state.next_number(task.kind)
 
           Exn.capture {
+            store.sync_permissions(context.dir)
+
             val isabelle_rev =
               sync(isabelle_repository, task.isabelle_rev, context.isabelle_dir)