Add capability to update `PS1` when loading the EESSI module #738

Job	Run time
eessi_container_script (no_cvmfs_mounts)	1m 59s
eessi_container_script (run)	2m 35s
eessi_container_script (help)	1m 8s
eessi_container_script (unionfs)	2m 8s
eessi_container_script (container)	1m 46s
eessi_container_script (shell)	1m 57s
eessi_container_script (listrepos_custom)	1m 40s
eessi_container_script (exec)	2m 16s
eessi_container_script (listrepos_default)	1m 34s
eessi_container_script (resume)	2m 14s
	19m 17s

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