Manual:$wgFileCacheDirectory

Details
If you switch on server-side file caching, then you can use this variable to specify where the cached pages will be saved on your server filesystem.

It's an absolute path, so you could specify somewhere under /tmp for example:

...but you can specify relative to other directory path variables...

This specifies a "cache" directory under the image uploads directory (normally called "images"). This is the default setting in fact.

If you are hosting multiple wikis from a shared source, you must ensure each has its own separate cache directory to avoid mis-serving pages with the same name (the Main Page, for example).