Bugfix: Fix rclone (shimmed by Scoop) and sftp not working on Windows

In #3602 a fix was introduced to address the problem of `rclone` prematurely
exiting when Ctrl+C is pressed on Windows. The solution was to create the
subprocess with its console detached from the restic console.

However, this solution failed when using `rclone` installed by Scoop or using
`sftp` with a passphrase-protected private key. We've now fixed this by using
a different approach to prevent Ctrl-C from passing down too early.

https://github.com/restic/restic/issues/3681
https://github.com/restic/restic/issues/3692
https://github.com/restic/restic/pull/3696
