fix EESSI prefix for complex prompts #962

Job	Run time
eessi_container_script (help)	1m 21s
eessi_container_script (resume)	2m 24s
eessi_container_script (unionfs)	2m 30s
eessi_container_script (listrepos_default)	1m 50s
eessi_container_script (container)	2m 18s
eessi_container_script (shell)	2m 20s
eessi_container_script (listrepos_custom)	2m 2s
eessi_container_script (exec)	2m 38s
eessi_container_script (run)	2m 44s
eessi_container_script (no_cvmfs_mounts)	2m 39s
	22m 46s

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