bug#36777: Guix Inferiors: Curious incorrect derivation output bug

[Carl Dong <contact@carldong.me>]

Hi all,

I've been working on improving the Guix build support on Bitcoin Core so that
it'll be ready to use for official releases.

One of the things that I wanted to do was to use a combination of channels and
inferiors to have reproducible builds across time. I have most of it set up, but
am running into some trouble. Here are the details:

I have a Guix channel set up at https://github.com/dongcarl/bitcoin-guix, where
I pin the Guix version to 6869b6635afd93872b8f0d9f2db0db4c0d765a86 in the
.guix-channel, and declare all my packages in packages.scm.

I have my manifest.scm here:
https://github.com/dongcarl/bitcoin/blob/2019-06-guix-channels-and-inferiors/contrib/guix/manifest.scm,
and it references the aforementioned Guix channel.

What I expect to happen is that now when I change my default profile's Guix
version by 'guix pull'ing, it won't affect the environment that's generated by
the manifest.scm. I've tested this with differing versions of Guix as my default
profile, and this seems to work with 7304d5623ab5cc35289cb1535cbd0d8a37691fac
and 7f1c69f5d32bee6b8b6b902a9ce445e04aa9d07d being my default profile. However,
I tried an older version b6dc08393e6a8313b88ce422fc3c1e4e9c0efc6f, and got the
following error:

```
guix environment: error: derivation `/gnu/store/r641vpqc9rfjhljf7rzfzwmkmpa642ls-info-dir.drv' has incorrect output `/gnu/store/q9hkdidycz3wq28xxgjq47bzx5s39k52-info-dir', should be `/gnu/store/z5hh2nl0h58b9f6hdxfwm00gjyxfcc3n-info-dir'
```

I remember my previous attempts at integrating channels and inferiors into our
workflow yielded a similar result, so I would like to know if this is a bug, and
how I might go about fixing this so that I can feel comfortable using inferiors
for our builds.

Nevertheless, thank you all for your hard work. The fact that we have inferiors
at all is marvelous!

Cheers,
Carl Dong
contact@carldong.me
"I fight for the users"