Use already instantiated cache object.

Instantiating a new caching object means `n98-magerun` misses the options used in `_initCache`

    #File: app/code/core/Mage/Core/Model/App.php
    protected function _initCache(array $cacheInitOptions = array())
    {
        $this->_isCacheLocked = true;
        $options = $this->_config->getNode('global/cache');
        if ($options) {
            $options = $options->asArray();
        } else {
            $options = array();
        }
        $options = array_merge($options, $cacheInitOptions);
        $this->_cache = Mage::getModel('core/cache', $options);
        $this->_isCacheLocked = false;
        return $this;
    }

If the user has default cache backend (i.e. redis) this may cause strange behavior, (see issue #417).  Unless it causes issue elsewhere, `n98-magerun` should use the already instantiated cache object.