bug#46389: Guix says it will download an output that is already downloaded

[pkill9]

> ‘guix build qtbase’ said it would download both the “out” and the
> “debug” output of qtbase, is that correct?

Yep

> It would be ideal if you could send precisely what’s on your terminal.
> 
> Thanks,
> Ludo’.

In this example, I've checked the store path
"/gnu/store/vpvnd6593mjncvyir2rbgp3k83cr7w0a-qtbase-5.15.2" exists, and
I've run `guix gc --delete
/gnu/store/f2s8ql1x9d9890qrrf9qq4nix3f5aii3-qtbase-5.15.2-debug`:

```
itsme@antelope ~> guix build -n qtbase
209.5 MB would be downloaded:
   /gnu/store/f2s8ql1x9d9890qrrf9qq4nix3f5aii3-qtbase-5.15.2-debug
   /gnu/store/vpvnd6593mjncvyir2rbgp3k83cr7w0a-qtbase-5.15.2
```
 I can't test it with `guix build qtbase` because my internet keeps
 cutting off while downloading qtbase-5.15.2-debug, but iirc
after it downloads the debug output, it just returns both store paths.