bug#46362: got unexpected path `Backtrace:' from substituter

[Zhu Zihao]

This issue is similar with https://issues.guix.gnu.org/45828. But it
still not fixed in 127a88d390417d5d7b1b4a18c1b69c7169dcaf34.

When you have multiple substituters, Guix will try to fetch narinfo from
the second or even third substituters, and it dies.

```
chino@asus-laptop:~$ guix build opencv 
--substitute-urls="https://mirror.sjtu.edu.cn/guix https://mirror.guix.org.cn 
https://mirror.c1r3u.xyz https://ci.guix.gnu.org";
substitute: updating substitutes from 'https://mirror.sjtu.edu.cn/guix'... 
100.0%
substitute: 
guix build: error: got unexpected path `Backtrace:' from substituter
```

If there's one substituter, it will work and do the substitution.
-- 
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  gpg --recv-keys D47A9C8B2AE3905B563D9135BE42B352A9F6821F

Zihao

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