Azure Pipelines supports bash tasks on Windows hosts due to it always
having Git for Windows included. To support this, the Git for Window
directory is added to the PATH environment to make the bash shell
available for execution. Unfortunately, this breaks CMake with the MinGW
generator, as it has sanity checks to verify that no bash executable is
in the PATH. So we can either remove Git for Windows from the path, but
then we're unable to execute bash jobs. Or we can add it to the path,
but then we're unable to execute CMake with the MinGW generator.

Let's re-model how we set the PATH environment. Instead of setting up
PATH for the complete build job, we now set a variable "BUILD_PATH" for
the job. This variable is only being used when executing CMake so that
it encounters a sanitizied PATH environment without GfW's bash shell.