bug#34444: Guix unable to download tarballs from github.com

[Leo Famulari]

I noticed that Guix cannot download any tarballs from github.com.

For example:

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$ guix build -S python-sphinx-gallery --no-substitutes                       
The following derivation will be built:
   
/gnu/store/qrlcbnin47j1i4slyc9j8hvpczp1l9n8-python-sphinx-gallery-0.1.13.tar.gz.drv
building 
/gnu/store/qrlcbnin47j1i4slyc9j8hvpczp1l9n8-python-sphinx-gallery-0.1.13.tar.gz.drv...

Starting download of 
/gnu/store/ks605lq7q0zf1agf84mslbk2cgdkg5hy-python-sphinx-gallery-0.1.13.tar.gz
>From https://github.com/sphinx-gallery/sphinx-gallery/archive/v0.1.13.tar.gz...
Throw to key `gnutls-error' with args `(#<gnutls-error-enum Resource 
temporarily unavailable, try again.> read_from_session_record_port)'.

[...]
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The server is advertising TLS 1.3 support.

Marius suggested that the Guix snapshot needs to be updated to include
commit e4ee84202633636b4c8cef4a332f0c74912a3b23 (download: Ask not to
use TLS 1.3.), and I am testing this now.