Bugfix: Never lock repository for `list locks`

The `list locks` command previously locked to the repository by default. This
had the problem that it wouldn't work for an exclusively locked repository and
that the command would also display its own lock file which can be confusing.

Now, the `list locks` command never locks the repository.

https://github.com/restic/restic/issues/1106
https://github.com/restic/restic/pull/3665