buildscripts/1.2.x: use 'cmd /c' to invoke the bash part of mumble-win32.cmd.
buildscripts/1.2.x: use 'cmd /c' to invoke the bash part of mumble-win32.cmd.

This should ensure that the errorlevel from the command is correctly
propagated back to us.

ccpgames/mumble-releng,ccpgames/mumble-releng