nix copy's output does not include the correct store URL when copying to an ssh-ng store

**Describe the bug**

```
post-build-hook: copying path '/nix/store/1njpv7qisg1l5h1jx3a6gwqi991fyam7-object-0.36.1' to 'ssh-ng://'...             
```

**Additional context**

Bisected to 3e9c3738d.

**Priorities**

Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).
