Previously, if /etc/guix/machines.scm was a dangling symlink, it would
be kept (because ‘file-exists?’ would return #f) and thus the following
‘symlink’ call would throw with EEXIST.
* gnu/services/base.scm (guix-machines-files-installation): Use ‘lstat’
rather than ‘file-exists?’.
Change-Id: I07c7eed842dacabbd19ae2a17ac3e59cf26e46b2