To be able to use Mercurial caching, one has to define a Mercurial installation and enable caching there. In case master and slave use a different OS, creating the cache clone on the master fails, simply because it's Linux and can't find C:\Program Files\TortoiseHg\hg.exe.

Therefor, enabling caching shouldn't be bound to specific mercurial installations. Instead, it should be a global option, maybe overridable at job level.