bug#34637: inferiors example yields error from guix package

[Martin Flack <martin.flack@gmail.com>]

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I'm getting an error trying the example here:
https://www.gnu.org/software/guix/manual/en/html_node/Inferiors.html

```
(use-modules (guix inferior) (guix channels)
             (srfi srfi-1))   ;for 'first'

(define channels
  ;; This is the old revision from which we want to
  ;; extract guile-json.
  (list (channel
         (name 'guix)
         (url "https://git.savannah.gnu.org/git/guix.git")
         (commit
          "65956ad3526ba09e1f7a40722c96c6ef7c0936fe"))))

(define inferior
  ;; An inferior representing the above revision.
  (inferior-for-channels channels))

;; Now create a manifest with the current "guile" package
;; and the old "guile-json" package.
(packages->manifest
 (list (first (lookup-inferior-packages inferior "guile-json"))
       (specification->package "guile")))
```

```
$ guix --version
guix (GNU Guix) 0964671086fb9e372894e788038aee75098e61a6
Copyright (C) 2019 the Guix authors
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html
>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
```

```
$ guix package -n -m example-inferior.scm
Updating channel 'guix' from Git repository at '
https://git.savannah.gnu.org/git/guix.git'...
Computing Guix derivation for 'x86_64-linux'... |
substitute: updating substitutes from 'https://bayfront.guixsd.org'...
100.0%
The following derivation will be built:
   /gnu/store/zxrm9a410iywcdm71crjbg2j4ggpvdbv-profile.drv
The following profile hook will be built:
   /gnu/store/gkhfa4z16x4336pbi1ha7njgdpm680yf-guix-package-cache.drv
guix package: error: exception thrown: #<condition &store-protocol-error
[message: "build of
`/gnu/store/zxrm9a410iywcdm71crjbg2j4ggpvdbv-profile.drv' failed" status:
100] 1f80de0>
```

I'm running guix on ubuntu 18.04.2.

I noticed bug 34402 was similar - I think I'm at the latest version of guix
today.

-- 

Martin

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