Fix an intermittent bootstrap failure after cache purges

[frederikmillingpytlick]

During tool_forcedcache_cache_factory::create_config_instance(), cache config loading can re-enter Moodle cache setup and reset the factory’s internal $configs array. The method then attempted to return $this->configs[$class], which could emit an undefined array key warning and return null, leading to:
Call to a member function get_definition_by_id() on null

This patch keeps the newly created config instance in a local variable, loads it, then re-registers it in $this->configs after load completes. That makes the method robust if $this->configs is reset during config generation.

Tested with:

php admin/cli/purge_caches.php
php admin/cli/purge_caches.php
php admin/cli/purge_caches.php

It doesn't seem to be an issue in the MOODLE_501_STABLE branch