Following rekado's friendly advice, I was able to downgrade guix with:
guix pull
--url=https://git.savannah.gnu.org/cgit/guix.git/snapshot/01049bb0c1f3f69afb8d1782f99ca5c0adaed946.tar.gz
--no-substitutes
where both options (--url and --no-substitutes) were neccessary. Now,
everything seems to work again: commands like `guix package -i` and
`guix system reconfigure` no longer get stuck at "updating list of
substitutes".
Thanks, rekado.
Still, I wonder
* what caused this issue;
* if anybody else is/was experiencing it; and
* when it will be fixed in master-branch (i.e. when I will be able to
`guix pull` and `guix pull` again successfully).
