bug#52846: Fetching "texlive" substitute from ci.guix.gnu.org fails

[Konrad Hinsen]

With

   $ guix describe
   Generation 11        déc. 27 2021 11:45:02   (current)
     guix 879a5cb
       repository URL: https://git.savannah.gnu.org/git/guix.git
       branch: master
       commit: 879a5cb7c45779ddd62699ae2d75a9e785d608ec

do:

   $ guix shell texlive
   hint: Consider passing the `--check' option once to make sure your shell 
does not clobber environment variables.

   substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
   The following derivations will be built:
      /gnu/store/6z9arnfp3c51iq99wddcbcal9zbbayav-profile.drv
      /gnu/store/js6cv4h83n60k8kh9vcgy2k0syfqm5yv-texlive-texmf-20210325.drv
      
/gnu/store/z5r60zy9vmydf44fdyyc7hgc6q93aahm-texlive-20210325-texmf.tar.xz.drv

   0,7 MB will be downloaded
   guix shell: error: integer expected from stream

The same error is shown for other attempts to access "texlive",
e.g. "guix package". Disabling substitutes solves the problem, for those
who have the patience to wait for texlive to build locally.

Cheers,
  Konrad.