Guix' SWH code hates me

[Leo Prikler]

I'm currently trying to update the package recipe for guile-wisp to the
newest version, but can't get it to download.  Inside a `guix
environment guix`, using `./pre-inst-env guix build`, I get the
following.

building /gnu/store/001p94i7q8kks3pngjah0b2v4fzl3npr-module-import-
compiled.drv...
[ 1/26] Loading './guix/base16.scm'...
[ 2/26] Loading './guix/base64.scm'...
[ 3/26] Loading './guix/build/download-nar.scm'...
[ 4/26] Loading './guix/build/download.scm'...
[ 5/26] Loading './guix/build/hg.scm'...
[ 6/26] Loading './guix/build/syscalls.scm'...
[ 7/26] Loading './guix/build/utils.scm'...
[ 8/26] Loading './guix/combinators.scm'...
[ 9/26] Loading './guix/ftp-client.scm'...
[10/26] Loading './guix/progress.scm'...
[11/26] Loading './guix/records.scm'...
[12/26] Loading './guix/serialization.scm'...
[13/26] Loading './guix/swh.scm'...
;;; Failed to autoload swh-download-directory in (guix swh):
;;; no code for module (json)

I know that Guix proper would have access to guile-json, but the pre-
inst-env one doesn't.  How do I make it so it does?