Bugfix: Fix "chmod not supported" errors when unlocking

Restic 0.18.0 introduced a bug that caused "chmod xxx: operation not supported"
errors to appear when unlocking with a stale lock, on a local file repository
that did not support chmod (like CIFS or WebDAV mounted via FUSE). Restic now
just doesn't bother calling chmod in that case on Unix, as it is unnecessary.

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