Tweak build system for windows

It can't find `cake`, even though it's on PATH - as `cake.cmd` though, so I added `.cmd` to it. `"shell": true` would also do it, if you prefer this.

And while I was editing it I also noticed the path override which obviously does not work on Windows.


Sadly, running `cake` still files a huge traceback for me, but that's a different thing.