Bugfix: Don't require `self-update --output` placeholder file

`restic self-update --output /path/to/new-restic` used to require that
new-restic was an existing file, to be overwritten. Now it's possible
to download an updated restic binary to a new path, without first
having to create a placeholder file.

https://github.com/restic/restic/issues/2491
https://github.com/restic/restic/pull/2937
