AppVeyor: Delay creation of file success.status
AppVeyor: Delay creation of file success.status

Previous CI tests failed due to git clone target directory already existing and not being empty. This may have been due to the creation of file success.status within the "init:" block. Therefore instead make this file within the "build_script:" block.
MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3