add symlink for var/log/wtmp

[trz42]

last accesses the file var/log/wtmp in a compatibility layer, for example, /cvmfs/pilot.eessi-hpc.org/2021.06/compat/linux/x86_64/var/log/wtmp.

That file however doesn't exist. It should be a symlink to the host's wtmp file, most likely, /var/log/wtmp.

One might also consider to symlink to /var/log, then however, existing content in /cvmfs/pilot.eessi-hpc.org/2021.06/compat/linux/x86_64/var/log requires a new location.

A temporary workaround for getting last to work would be to run

last -f /var/log/wtmp

[amadio]

According to the man page, you may want to also link /var/log/btmp.

[bedroge]

Should be fixed by #199.