Bugfix: Avoid spurious `chmod` errors on certain file backends

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

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