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

[raid5atemyhomework]

Hi all,

In case it's useful, here's my `traceroute`, would this be helpful for Guix?

I erased some internal IP addresses and omit a few bytes off the source 
country. After hop 25 `traceroute` couldn't find anything anymore. I annotated 
the IP addresses to country map.

```
$ sudo `which traceroute` 141.80.181.40
traceroute to 141.80.181.40 (141.80.181.40), 64 hops max
  1   <LAN>  3.140ms  0.947ms  0.981ms
  2   <ISP INTERNAL>  2.527ms  1.571ms  1.416ms
  3   <ISP INTERNAL>  4.318ms  4.705ms  3.680ms
  4   <COUNTRY>.4.16  2.605ms  1.856ms  1.963ms
  5   <COUNTRY>.1.205  2.177ms  2.087ms  1.876ms
  6   <COUNTRY>.3.54  29.090ms  30.237ms  28.982ms
  7   203.208.168.129  29.200ms  29.497ms  30.863ms     # Singapore Singapore 
Telecommunications Pte Ltd
  8   203.208.152.82  29.295ms  29.231ms  29.227ms      # Singapore SingTel 
Internet Exchange
  9   203.208.182.249  30.769ms  31.872ms  47.615ms     # Singapore Singapore 
Telecommunications Pte Ltd
 10   203.208.183.133  30.271ms  30.713ms  31.635ms     # Singapore Singapore 
Telecommunications Pte Ltd
 11   203.208.158.178  363.911ms  301.844ms  204.896ms  # Singapore SingTel 
Internet Exchange
 12   203.208.172.234  213.664ms  298.038ms  213.798ms  # Singapore Singapore 
Telecommunications Pte Ltd
 13   *  *  *
 14   *  *  *
 15   *  *  *
 16   *  *  *
 17   *  *  *
 18   64.125.28.36  335.579ms  499.267ms  411.078ms     # Netherlands Zayo 
Bandwidth
 19   *  *  *
 20   64.125.28.36  349.449ms  347.413ms  432.726ms     # Netherlands Zayo 
Bandwidth
 21   80.81.193.222  334.576ms  334.150ms  335.696ms    # Germany DE-CIX 
Management GmbH
 22   64.125.26.173  343.767ms  344.376ms  344.544ms    # Germany Zayo Bandwidth
 23   188.1.238.78  375.499ms  346.705ms  417.986ms     # Germany Verein zur 
Foerderung eines Deutschen Forschungsnetzes e.V.
 24   141.80.238.11  339.792ms  383.421ms  406.579ms    # Germany Campus 
Berlin-Buch GmbH
 25   188.1.146.210  410.225ms  334.844ms  335.196ms    # Germany Verein zur 
Foerderung eines Deutschen Forschungsnetzes e.V.
```

The big increase in ping times is somewhere inside Singapore, which seems like 
a reasonable assumption that it's what is throttling bandwidth. A quick 
research shows Singapore Telecommunications Pte Ltd is the same as SingTel 
Internet Exchange.

Are there any other mirror substitute servers aside from `ci.guix.gnu.org`?

Thanks
raid5atemyhomework