add pre_prepare hook for Pytorch which solves executable stack errors #811

Job	Run time
eessi_container_script (run)	3m 29s
eessi_container_script (listrepos_default)	2m 48s
eessi_container_script (help)	2m 15s
eessi_container_script (exec)	4m 7s
eessi_container_script (listrepos_custom)	3m 11s
eessi_container_script (shell)	5m 22s
eessi_container_script (container)	2m 58s
eessi_container_script (unionfs)	2m 49s
eessi_container_script (resume)	3m 8s
eessi_container_script (no_cvmfs_mounts)	3m 8s
	33m 15s

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