Bugfix: Use mode 0700 for repository directories created over SFTP

When creating a repository over SFTP, restic created the repository directories
with the SFTP server's default permissions, often 0755, rather than the 0700
permissions it uses for local repositories.

Restic now creates these directories with 0700 permissions.

https://github.com/restic/restic/issues/4447
https://github.com/restic/restic/pull/21817
