Fixes a bug whereby running 'guix system init config.scm /mnt' twice
would, on the second run, change timestamps in /mnt/gnu/store from the
Epoch to now. This is because the 'register-path' call would bypass the
'reset-timestamps' phase altogether in that case, as a consequence of
commit bb3b6ccb05.
Reported by Christopher Baines.
* guix/scripts/system.scm (install): When TARGET/var/guix exists, delete
it. As a side-effect, this ensures that later on, the 'register-path'
call invokes 'reset-timestamps' on all the copied store items.
6c843907e9