Fix post-reviews.py to find `rbt.cmd` on Windows

The script cannot find `rbt` since the command on Windows is `rbt.cmd` and `CreateProcess` (used by Python's Popen implementation) will not search through suffixes (PowerShell, however, does do this, so don't be confused when PowerShell can find `rbt`). So the script needs to use `rbt.cmd` when on Windows.