Bugfix: Mark files as readonly when using the SFTP backend

Files created by the SFTP backend previously allowed writes to those files.
Restic now restricts the file permissions on SFTP backend to readonly.
This change only has an effect for sftp servers with support for the chmod operation.

https://github.com/restic/restic/issues/5487
https://github.com/restic/restic/pull/5497