GC breaks on multisite when visiting WPSC admin on non-main site

Simply visiting the WP Super Cache admin causes wp-cache-config.php to be written. If you have a (subdir based) multisite installation, and you visit WPSC admin for a particular non-main site to, say, clear the cache on that blog, $wp_cache_home_path will be overwritten with the more specific site path, causing GC breakage for all other sites.

Seems like the code around https://github.com/Automattic/wp-super-cache/blob/master/wp-cache.php#L937 should have a multisite check and use the main site's path. Otherwise, some sort of "if multisite, and not on main site, don't write wp-cache-config.php file" logic is needed to mitigate this issue.