MDL-77837 cron: Ensure user is set when running tasks

We should be proactive in ensuring that the environment is clean when
running a task. We already ensure that we have a clean renderer and
other parts of the output chain, but we were not setting a clean user.

This change adds a call to setup the cron user before each task is
actually executed.

lib/cronlib.php

@@ -247,6 +247,10 @@ function cron_run_inner_scheduled_task(\core\task\task_base $task) {
     $predbqueries = null;
     $predbqueries = $DB->perf_get_queries();
     $pretime = microtime(1);
+
+    // Ensure that we have a clean session with the correct cron user.
+    cron_setup_user();
+
     try {
         get_mailer('buffer');
         cron_prepare_core_renderer();
@@ -333,6 +337,10 @@ function cron_run_inner_adhoc_task(\core\task\adhoc_task $task) {
         }
 
         cron_setup_user($user);
+    } else {
+        // No user specified, ensure that we have a clean session with the correct cron user.
+        cron_setup_user();
+
     }
 
     try {