Bugfix: Avoid spurious `chmod` errors when unlocking on file backends

On filesystems that do not support `chmod` (for example CIFS or FUSE-mounted WebDAV),
restic 0.18.0 failed to remove stale locks with the error `chmod …: operation not supported`.
This has been fixed.

https://github.com/restic/restic/issues/5595
https://github.com/restic/restic/pull/5596
