Hello Olivier,
In Windows, I have done a similar procedure using below syntax.
cmd /C start /wait <somebatchfile>
This basically puts the process to wait.
In your case, using exec or similar command might help. (but the problem with exec is, it forks subprocess without any variables shared between the shell environment).
The easiest option is to use below. (find how long your magic script takes, and insert that as a sleep command, this makes Snap Creator to wait)
POST_CLONE_CREATE_CMD01=CLONETARGET: command
POST_CLONE_CREATE_CMD02=CLONETARGET: sleep 120
Please let me know if this helps.
Thanks,
Siva Ramanathan