Substitute not downloading

[Greg Hogan]

I have an up-to-date Guix but am unable to fetch the substitute from
  http://ci.guix.gnu.org/build/1332269/details

Are substitutes only available after the evaluation has completed?

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$ guix describe
Generation 9    May 09 2023 15:18:11    (current)
  guix c1ffe2f
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: c1ffe2f21bd1b9ba6bd527bbabe130144a69af71


guix install ungoogled-chromium
The following package will be installed:
   ungoogled-chromium 112.0.5615.165-1

The following derivations will be built:
  /gnu/store/lxix0vxmgdvvddk6wl2fdag7vrqhklky-profile.drv
  
/gnu/store/92l92ljnq20q09bvjxlw42n5gl8shf38-ungoogled-chromium-112.0.5615.165-1.drv

611.0 MB will be downloaded
 chromium-112.0.5615.165.tar.xz  583.6MiB
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