add pre_prepare hook for Pytorch which solves executable stack errors #808

Job	Run time
eessi_container_script (resume)	2m 49s
eessi_container_script (container)	2m 24s
eessi_container_script (listrepos_default)	2m 6s
eessi_container_script (run)	2m 40s
eessi_container_script (listrepos_custom)	1m 57s
eessi_container_script (shell)	2m 15s
eessi_container_script (no_cvmfs_mounts)	2m 35s
eessi_container_script (help)	3m 31s
eessi_container_script (exec)	2m 39s
eessi_container_script (unionfs)	4m 46s
	27m 42s

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