generators: when creating symlinks, silently ignore existing links in one more place

generators: when creating symlinks, silently ignore existing links in one more place

After the update to systemd 257.7 in Fedora, there are reports that we fail to
create a symlink:
  systemd-gpt-auto-generator[585]: Failed to create symlink /run/systemd/generator/local-fs.target.wants/systemd-fsck-root.service: File exists
  (sd-exec-[574]: /usr/lib/systemd/system-generators/systemd-gpt-auto-generator failed with exit status 1.

I guess that some other generator created the symlink. We silently ignore
EEXIST in similar codepaths, so add that in one more place. (The target of the
symlink doesn't really matter. The name of the link matters. So something like
symlink_idempotent would not be better. For example, a different generator
might use a slightly different target path, and symlink_idempotent would be too
strict.)