bug#46942: ci.guix.gnu.org is slow from my system

[raid5atemyhomework]

Here's another error!

For *this* instance notice the very slow download speed; other downloads got up 
to 2MiB/s.  If my understanding is correct the SJTUG server is effectively a 
caching proxy, meaning that the low download speed here probably means that the 
SJTUG server itself is downloading from the Berlin Cuirass.  As noted before, 
often an issue with using ci.guix.gnu.org is that for large multi-megabyte 
substitutes sometimes the Berlin server will just disconnect and EOF early, so 
I suspect something similar is happening to the SJTUG server.

```
downloading from 
https://mirror.sjtu.edu.cn/guix/nar/lzip/6fa3h0n957hr2nbd05ygjnk4idjq122q-ffmpeg-4.3.2
 ...
 ffmpeg-4.3.2  8.8MiB                                                           
                                                                     13KiB/s 
05:05 [#######           ]  44.2%
Backtrace:
In guix/store/deduplication.scm:
    227:2 19 (dump-file/deduplicate "/gnu/store/6fa3h0n957hr2nbd05y…" …)
In ice-9/ports.scm:
   463:17 18 (call-with-output-file _ _ #:binary _ #:encoding _)
In guix/store/deduplication.scm:
   232:10 17 (_ _)
In guix/serialization.scm:
    261:6 16 (dump _)
   247:20 15 (dump #<input: string 7fb2d8c83cb0> #<output: string 7…> …)
In unknown file:
          14 (get-bytevector-n! #<input: string 7fb2d8c83cb0> # 0 #)
In guix/store/deduplication.scm:
   203:22 13 (read! #vu8(85 110 107 110 111 119 110 32 112 105 99 …) …)
In unknown file:
          12 (get-bytevector-n! #<input: string 7fb2d8ed4d90> # 0 #)
In gcrypt/hash.scm:
   223:13 11 (read! #vu8(85 110 107 110 111 119 110 32 112 105 99 …) …)
In unknown file:
          10 (get-bytevector-n! #<input: string 7fb2d8ed4e70> # 0 #)
In lzlib.scm:
    501:4  9 (lzread! #<lz-decoder 7fb2d8c6bbf0> #<input: string 7f…> …)
In unknown file:
           8 (get-bytevector-n #<input: string 7fb2d8ed4ee0> 65537)
In guix/progress.scm:
   358:30  7 (read! _ _ _)
In unknown file:
           6 (get-bytevector-n! #<input: string 7fb2d8c832a0> # 0 #)
In web/response.scm:
     95:2  5 (read! _ _ _)
In ice-9/boot-9.scm:
  1669:16  4 (raise-exception _ #:continuable? _)
  1669:16  3 (raise-exception _ #:continuable? _)
  1764:13  2 (_ #<&compound-exception components: (#<&error> #<&irri…>)
  1669:16  1 (raise-exception _ #:continuable? _)
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
Throw to key `bad-response' with args `("EOF while reading response body: ~a 
bytes of ~a" (4112173 9196598))'.
Backtrace:
In ice-9/boot-9.scm:
  1736:10  4 (with-exception-handler _ _ #:unwind? _ # _)
In unknown file:
           3 (apply-smob/0 #<thunk 7fb2dec76520>)
In ice-9/boot-9.scm:
    718:2  2 (call-with-prompt _ _ #<procedure default-prompt-handle…>)
In ice-9/eval.scm:
    619:8  1 (_ #(#(#<directory (guile-user) 7fb2dec79c80>)))
In guix/ui.scm:
  2164:12  0 (run-guix-command _ . _)

guix/ui.scm:2164:12: In procedure run-guix-command:
Throw to key `bad-response' with args `("EOF while reading response body: ~a 
bytes of ~a" (4112173 9196598))'.
substitution of /gnu/store/6fa3h0n957hr2nbd05ygjnk4idjq122q-ffmpeg-4.3.2 failed
guix upgrade: error: some substitutes for the outputs of derivation 
`/gnu/store/9xx9kbbjals2y7llhak65fnnyfh9rkyq-gnome-tweaks-3.34.1.drv' failed 
(usually happens due to networking issues); try `--fallback' to build 
derivation from source
```

Thanks
raid5atemyhomework