ci/buildsystem: ensure PRE_FLASH_CHECK_SCRIPT is not exported
ci/buildsystem: ensure PRE_FLASH_CHECK_SCRIPT is not exported

OlegHahm/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,miri64/RIOT,miri64/RIOT,yogo1212/RIOT,kYc0o/RIOT,OlegHahm/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,yogo1212/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,basilfx/RIOT,yogo1212/RIOT,jasonatran/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,basilfx/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,jasonatran/RIOT,yogo1212/RIOT,kYc0o/RIOT,kYc0o/RIOT,jasonatran/RIOT,kYc0o/RIOT,kaspar030/RIOT,yogo1212/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,ant9000/RIOT,basilfx/RIOT,kaspar030/RIOT,ant9000/RIOT,basilfx/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT