add pre_prepare hook for Pytorch which solves executable stack errors #827

Job	Run time
eessi_container_script (unionfs)	2m 41s
eessi_container_script (help)	1m 31s
eessi_container_script (listrepos_default)	1m 43s
eessi_container_script (listrepos_custom)	1m 35s
eessi_container_script (resume)	2m 5s
eessi_container_script (shell)	2m 9s
eessi_container_script (container)	2m 36s
eessi_container_script (no_cvmfs_mounts)	2m 12s
eessi_container_script (exec)	2m 18s
eessi_container_script (run)	2m 14s
	21m 4s

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