Respect cache_path config overrides

We use an [officially sanctioned method][1] to override the system
cache path in our site's config files. EE Debug Toolbar ignores this
setting when creating its cache file, and attempts to derive the cache
location using the `APPPATH` constant. Since the assumed path is
invalid, this results in PHP errors on the Add-ons -> Extensions page
in the control panel.

This modification updates EE Debug Toolbar's constructor method to
reference the cache path defined by the system config override (if
set), and defaults to the default location otherwise.

[1]:
https://ellislab.com/expressionengine/user-guide/general/system_configur
ation_overrides.html#cache-path