After a recent guix pull, I'm getting tracebacks when attempting to run
guix challenge.

I tried switching subsititues to a local caching proxy, not sure if that
could possibly break anything. But after switching back to using the
substitutes directly, the problem still persists.

$ guix --version
guix (GNU guix) 1fa37d1b55b1d25a9d20c7b50a531b763e7a1398

$ guix challenge --substitute-urls="https://berlin.guixsd.org https://mirror.hydra.gnu.org" linux-libre
Backtrace:
          12 (primitive-load "/gnu/store/sy84f3kbjj2f2nl1ljbdcmsqrz8…")
In guix/ui.scm:
  1452:12 11 (run-guix-command _ . _)
In ice-9/boot-9.scm:
    837:9 10 (catch _ _ #<procedure 7f185b219070 at guix/ui.scm:513…> …)
    837:9  9 (catch _ _ #<procedure 7f185b219088 at guix/ui.scm:611…> …)
    837:9  8 (catch _ _ #<procedure 7f1857f1bd70 at guix/scripts/ch…> …)
In guix/scripts/challenge.scm:
   304:13  7 (_)
In guix/store.scm:
  1444:24  6 (run-with-store _ _ #:guile-for-build _ #:system _ # _)
In guix/scripts/challenge.scm:
   305:15  5 (_ _)
   149:34  4 (_ _)
In srfi/srfi-1.scm:
   679:15  3 (append-map _ _ . _)
   592:17  2 (map1 ("https://berlin.guixsd.org"; "https://mirror.hy…"))
In guix/scripts/substitute.scm:
   725:23  1 (lookup-narinfos "https://berlin.guixsd.org"; _)
   697:16  0 (fetch-narinfos "https://berlin.guixsd.org"; ("/gnu/sto…"))

guix/scripts/substitute.scm:697:16: In procedure fetch-narinfos:
guix/scripts/substitute.scm:697:16: Wrong type to apply: #<syntax-transformer uri?>


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  vagrant