improve run_cmake.py on Windows
improve run_cmake.py on Windows

Summary:
Update the generated `run_cmake.py` script to use `subprocess.run()` instead
of `os.execve()`.  The `os.execve()` call doesn't really do what we want on
Windows: this causes the script to exit while CMake is still running,
resulting in confusing output.  During the build step it also did not work
correctly with `vcvarsall.bat`, and using `subprocess` also solves this.

Reviewed By: wez

Differential Revision: D17493897

fbshipit-source-id: e0477627fc1824b0efcb1fa5a782d207853bcae8

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly