bug#40713: Guix fails to build a system when specifying an inferior package for the kernel

[pkill9]

I'm getting an error when trying to build a Guix system on the latest
Guix revision (commit ea6594e08e2724c64bc07724a07479fc1633dede):

```
Backtrace:
           1 (primitive-load "/root/.config/guix/current/bin/guix")
In guix/ui.scm:
  1936:12  0 (run-guix-command _ . _)

guix/ui.scm:1936:12: In procedure run-guix-command:
In procedure %package-replacement-real: Wrong type argument:
#<inferior-package linux@5.5.9 7f659c641150>
```

I can't find '%package-replacement-real' anywhere in the source.