bug#52452: Guix related downloads from ftp.gnu.org are slow over IPv6

[Christopher Baines]

900 times slower by my measurements. I noticed this when downloading the
release .tar.xz:

→ wget -4 https://ftp.gnu.org/gnu/guix/guix-binary-1.3.0.x86_64-linux.tar.xz
--2021-12-12 14:07:29--  
https://ftp.gnu.org/gnu/guix/guix-binary-1.3.0.x86_64-linux.tar.xz
Resolving ftp.gnu.org (ftp.gnu.org)... 209.51.188.20
Connecting to ftp.gnu.org (ftp.gnu.org)|209.51.188.20|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 91030312 (87M) [application/x-xz]
Saving to: ‘guix-binary-1.3.0.x86_64-linux.tar.xz’

guix-binary-1.3.0.x 100%[===================>]  86.81M  29.4MB/s    in 2.9s    

2021-12-12 14:07:32 (29.4 MB/s) - ‘guix-binary-1.3.0.x86_64-linux.tar.xz’ saved 
[91030312/91030312]


→ wget -6 https://ftp.gnu.org/gnu/guix/guix-binary-1.3.0.x86_64-linux.tar.xz
--2021-12-12 14:07:38--  
https://ftp.gnu.org/gnu/guix/guix-binary-1.3.0.x86_64-linux.tar.xz
Resolving ftp.gnu.org (ftp.gnu.org)... 2001:470:142:3::b
Connecting to ftp.gnu.org (ftp.gnu.org)|2001:470:142:3::b|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 91030312 (87M) [application/x-xz]
Saving to: ‘guix-binary-1.3.0.x86_64-linux.tar.xz.1’

guix-binary-1.3.0.x86_64-linux.tar.xz.1 
100%[===================================================================>]  
86.81M  32.4KB/s    in 43m 36s 

2021-12-12 14:51:14 (34.0 KB/s) - ‘guix-binary-1.3.0.x86_64-linux.tar.xz.1’ 
saved [91030312/91030312]

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