Failed to autoload read-substitute-urls in (guix scripts discover)

[Philip McGrath]

On a Debian Bookworm system, most Guix commands are failing for me with errors 
like the following unless I supply --no-substitutes:
```
philip@avalon:~$ guix build hello
substitute: ;;; Failed to autoload read-substitute-urls in (guix scripts 
discover):
substitute: ;;; no code for module (guix scripts discover)
substitute: Backtrace:
substitute: In ice-9/boot-9.scm:
substitute:   1752:10 15 (with-exception-handler _ _ #:unwind? _ # _)
substitute: In unknown file:
substitute:           14 (apply-smob/0 #<thunk 7f6408f132e0>)
substitute: In ice-9/boot-9.scm:
substitute:     724:2 13 (call-with-prompt _ _ #<procedure 
default-prompt-handle…>)
substitute: In ice-9/eval.scm:
substitute:     619:8 12 (_ #(#(#<directory (guile-user) 7f6408f18c80>)))
substitute: In guix/ui.scm:
substitute:    2275:7 11 (run-guix . _)
substitute:    2217:7 10 (run-guix-command substitute "--query")
substitute: In ice-9/boot-9.scm:
substitute:   1747:15  9 (with-exception-handler #<procedure 7f6400cb93f0 at 
ic…> …)
substitute:   3327:17  8 (resolve-interface (guix scripts substitute) #:select 
_ …)
substitute: In ice-9/threads.scm:
substitute:     390:8  7 (_ _)
substitute: In ice-9/boot-9.scm:
substitute:   3253:13  6 (_)
substitute: In ice-9/threads.scm:
substitute:     390:8  5 (_ _)
substitute: In ice-9/boot-9.scm:
substitute:   3544:20  4 (_)
substitute:    2836:4  3 (save-module-excursion _)
substitute:   3564:26  2 (_)
substitute: In unknown file:
substitute:            1 (primitive-load-path "guix/scripts/substitute" 
#<proced…>)
substitute: In guix/scripts/substitute.scm:
substitute:    753:33  0 (_)
substitute: 
substitute: guix/scripts/substitute.scm:753:33: error: read-substitute-urls: 
unbound variable
guix build: error: `/usr/bin/guix substitute' died unexpectedly
philip@avalon:~$ command -v guix
/home/philip/.config/guix/current/bin/guix
```

Trying `guix pull --no-substitutes` works, but with the obvious downsides.

It seems like there's something wrong with the Guile load path, but I don't 
understand what. I've attached the environment variables in effect, if that 
helps.

It's possible that this could be somehow related to 
<https://lists.gnu.org/archive/html/guix-devel/2023-05/msg00235.html>, but I've 
applied the same workaround I use on a Kubuntu 22.10 system that doesn't have 
this problem.

-Philip

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