We currently unconditionally override the PATH variable with a custom
path with the main intent of making available our own custom MinGW
installation. This worked quite well so far, but is heavily dependent on
the machine we're running this on. And naturally, it fails on the new
Windows machines we need to upgrade to, as tools like CMake are not
contained in the path we currently set up.

Fix this by remodeling the way we set up the PATH environment. Instead
of overriding it completely, we now override it only when executing
the CMake build.