Add a top-lever dir init/modules/EESSI containing a symlink to the 2023.06 modulefile

[bedroge]

Tested it in a transaction that I aborted/didn't publish, this is the result:

[bob@multixscale-stratum0 ~]$ sudo ls -lR /cvmfs/software.eessi.io/init
/cvmfs/software.eessi.io/init:
total 0
drwxr-xr-x. 3 root root 19 Sep  5 13:24 modules

/cvmfs/software.eessi.io/init/modules:
total 0
drwxr-xr-x. 2 root root 25 Sep  5 13:24 EESSI

/cvmfs/software.eessi.io/init/modules/EESSI:
total 0
lrwxrwxrwx. 1 root root 72 Sep  5 13:24 2023.06.lua -> /cvmfs/software.eessi.io/versions/2023.06/init/modules/EESSI/2023.06.lua